Magnetic recording medium, magnetic recording/reproducing device, and magnetic recording medium cartridge

ABSTRACT

A magnetic recording medium that can exhibit good traveling performance during use is provided. This magnetic recording medium is a tape-shaped magnetic recording medium, and includes a substrate including a polyester as a main component, a base layer disposed on the substrate, a magnetic layer disposed on the base layer, and a back layer disposed on a side of the substrate opposite to the base layer. A surface of the back layer opposite to the substrate has a kurtosis of 2.0 or more. On a surface of the magnetic layer, recesses having a depth of 20% or more of the average thickness of the magnetic layer are formed at a ratio of 10 or more and 200 or less per 1600 μm2. A surface of the magnetic layer has arithmetic average roughness Ra of 2.5 nm or less. The entire magnetic recording medium has a BET specific surface area of 3.5 m2/g or more in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried.

TECHNICAL FIELD

The present disclosure relates to a magnetic recording medium, and amagnetic recording/reproducing device and a magnetic recording mediumcartridge using the magnetic recording medium.

BACKGROUND ART

A tape-shaped magnetic recording medium is widely used for storingelectronic data. For example, Patent Document 1 describes that a surfaceof a magnetic layer is smoothed in order to improve electromagneticconversion characteristics of a magnetic recording medium. Furthermore,this document describes that a lubricant is added to the magnetic layerin order to suppress friction caused by contact between the magneticrecording medium and a head.

CITATION LIST Patent Document

Patent Document 1: Japanese Patent Application Laid-Open No. 2006-65953

SUMMARY OF THE INVENTION

The tape-shaped magnetic recording medium is housed in, for example, amagnetic recording cartridge. In order to further increase the recordingcapacity per magnetic recording cartridge, it is considered to reducethe total thickness of the magnetic recording medium housed in themagnetic recording cartridge, and to increase the length of the magneticrecording medium per magnetic recording cartridge (so-called tapelength). However, a magnetic recording medium having a small totalthickness may have poor traveling stability. In particular, in a casewhere repeated recording and/or reproduction are/is performed, amagnetic recording medium having a small total thickness may change asurface state thereof (particularly, a surface state thereof related tofriction), and may deteriorate traveling stability thereof.

Therefore, a magnetic recording medium having a small total thicknessand excellent traveling stability is desired even after repeatedrecording and reproducing operations are performed.

A magnetic recording medium according to an embodiment of the presentdisclosure is a tape-shaped magnetic recording medium, and includes asubstrate, a base layer disposed on the substrate, a magnetic layerdisposed on the base layer, and a back layer disposed on a side of thesubstrate opposite to the base layer. The substrate includes a polyesteras a main component. The kurtosis of a surface of the back layeropposite to the substrate is 2.0 or more. On a surface of the magneticlayer, recesses having a depth of 20% or more of the average thicknessof the magnetic layer are formed at a ratio of 10 or more and 200 orless per 1600 μm², and the arithmetic average roughness Ra of thesurface of the magnetic layer is 2.5 nm or less. The base layer and themagnetic layer each include a lubricant, and the BET specific surfacearea of the entire magnetic recording medium is 3.5 m²/g or more in astate where the lubricant has been removed from the magnetic recordingmedium and the magnetic recording medium has been dried. The squarenessratio in the perpendicular direction is 65% or more. The averagethickness of the magnetic layer is 90 nm or less. The average thicknessof the magnetic recording medium is 5.6 μm or less.

A magnetic recording/reproducing device according to an embodiment ofthe present disclosure includes a feeding unit that can sequentiallyfeed out the magnetic recording medium described above, a winding unitthat can wind up the magnetic recording medium fed out from the feedingunit, and a magnetic head that can write information on the magneticrecording medium and can read out information from the magneticrecording medium while being in contact with the magnetic recordingmedium traveling from the feeding unit toward the winding unit.

In the magnetic recording medium and the magnetic recording/reproducingdevice according to an embodiment of the present disclosure, the BETspecific surface area of the entire magnetic recording medium is 3.5m²/g or more. Therefore, the lubricant is stably supplied to a surfaceof the magnetic recording medium. Furthermore, since the kurtosis of asurface of the back layer opposite to the substrate is 2.0 or more,winding deviation of the magnetic recording medium hardly occurs.Moreover, since recesses are formed on a surface of the magnetic layerwith a predetermined surface density, contact between the surface of themagnetic layer and the head is maintained well during traveling.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a magnetic recording mediumaccording to an embodiment of the present disclosure.

FIG. 2A is a first explanatory diagram illustrating a method formeasuring the surface density of recesses in the magnetic recordingmedium illustrated in FIG. 1 .

FIG. 2B is a second explanatory diagram illustrating a method formeasuring the surface density of recesses in the magnetic recordingmedium illustrated in FIG. 1 .

FIG. 2C is a third explanatory diagram illustrating a method formeasuring the surface density of recesses in the magnetic recordingmedium illustrated in FIG. 1 .

FIG. 3A is a schematic explanatory diagram illustrating a layout of databands and servo bands in the magnetic recording medium illustrated inFIG. 1 .

FIG. 3B is a schematic explanatory diagram illustrating one of the databands illustrated in FIG. 3A in an enlarged manner.

FIG. 4 is a cross-sectional view schematically illustrating across-sectional structure of an ε-iron oxide particle included in themagnetic layer illustrated in FIG. 1 .

FIG. 5 is a graph illustrating an example of an SFD curve of themagnetic recording medium illustrated in FIG. 1 .

FIG. 6 is a schematic diagram of a recording/reproducing device usingthe magnetic recording medium illustrated in FIG. 1 .

FIG. 7 is a cross-sectional view schematically illustrating across-sectional structure of an ε iron oxide particle as a modification.

FIG. 8 is a cross-sectional view of a magnetic recording medium asanother modification.

FIG. 9 is a schematic diagram illustrating a method for measuring acoefficient of dynamic friction.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present disclosure will be describedin detail with reference to the drawings. Note that the description willbe made in the following order.

1. One embodiment

1-1. Configuration of magnetic recording medium

1-2. Method for manufacturing magnetic recording medium

1-3. Configuration of recording/reproducing device

1-4. Effect

2. Modification

<1. One Embodiment>

[1-1 Configuration of Magnetic Recording Medium 10]

FIG. 1 illustrates a cross-sectional configuration example of a magneticrecording medium 10 according to an embodiment of the presentdisclosure. As illustrated in FIG. 1 , the magnetic recording medium 10has a laminated structure in which a plurality of layers is laminated.Specifically, the magnetic recording medium 10 includes a longtape-shaped substrate 11, a base layer 12 disposed on one main surface11A of the substrate 11, a magnetic layer 13 disposed on the base layer12, and a back layer 14 disposed on the other main surface 11B of thesubstrate 11. A surface 13S of the magnetic layer 13 is a surface onwhich a magnetic head travels while being in contact with the surface13S. Note that the base layer 12 and the back layer 14 are disposed asnecessary and may be omitted. Note that the average thickness of themagnetic recording medium 10 is preferably 5.6 μm or less, for example.

The magnetic recording medium 10 has a long tape shape, and travels inits own longitudinal direction during recording and reproducingoperations. The magnetic recording medium 10 is preferably used for arecording/reproducing device including a ring type head as a recordinghead, for example.

(Substrate 11)

The substrate 11 is a nonmagnetic support for supporting the base layer12 and the magnetic layer 13. The substrate 11 has a long film shape. Anupper limit value of the average thickness of the substrate 11 ispreferably 4.2 μm or less, and more preferably 4.0 μm or less. When theupper limit value of the average thickness of the substrate 11 is 4.2 μmor less, the recording capacity that can be recorded in one datacartridge can be increased as compared to a general magnetic recordingmedium. A lower limit value of the average thickness of the substrate 11is preferably 3 μm or more, and more preferably 3.2 μm or more. When thelower limit value of the average thickness of the substrate 11 is 3 μmor more, a decrease in strength of the substrate 11 can be suppressed.

The average thickness of the substrate 11 is determined as follows.First, the magnetic recording medium 10 having a width of ½ inches isprepared and cut into a length of 250 mm to manufacture a sample.Subsequently, layers of the sample other than the substrate 11, that is,the base layer 12, the magnetic layer 13, and the back layer 14 areremoved with a solvent such as methyl ethyl ketone (MEK) or dilutehydrochloric acid. Next, using a laser hologauge (LGH-110C) manufacturedby Mitutoyo Corporation as a measuring device, the thickness of thesubstrate 11 as a sample is measured at five or more points. Thereafter,the measured values are simply averaged (arithmetically averaged) tocalculate the average thickness of the substrate 11. Note that themeasurement points are randomly selected from the sample.

The substrate 11 includes, for example, a polyester as a main component.The substrate 11 may include at least one of a polyolefin, a cellulosederivative, a vinyl-based resin, and another polymer resin in additionto a polyester. In a case where the substrate 11 includes two or more ofthe materials described above, the two or more materials may be mixed,copolymerized, or laminated.

The polyester included in the substrate 11 includes, for example, atleast one of polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polybutylene terephthalate (PBT), polybutylene naphthalate (PBN),polycyclohexylenedimethylene terephthalate (PCT),polyethylene-p-oxybenzoate (PEB), and polyethylenebisphenoxycarboxylate.

The polyolefin included in the substrate 11 includes, for example, atleast one of polyethylene (PE) and polypropylene (PP). The cellulosederivative includes, for example, at least one of cellulose diacetate,cellulose triacetate, cellulose acetate butyrate (CAB), and celluloseacetate propionate (CAP). The vinyl-based resin includes, for example,at least one of polyvinyl chloride (PVC) and polyvinylidene chloride(PVDC).

The other polymer resin included in the substrate 11 includes, forexample, at least one of polyamide or nylon (PA), aromatic polyamide oraramid (aromatic PA), polyimide (PI), aromatic polyimide (aromatic PI),polyamide imide (PAI), aromatic polyamide imide (aromatic PAI),polybenzoxazole (PBO) such as ZYLON (registered trademark), polyether,polyether ketone (PEK), polyether ester, polyether sulfone (PES),polyether imide (PEI), polysulfone (PSF), polyphenylene sulfide (PPS),polycarbonate (PC), polyarylate (PAR), and polyurethane (PU).

(Magnetic Layer 13)

The magnetic layer 13 is a recording layer for recording a signal. Themagnetic layer 13 includes, for example, magnetic powder, a binder, anda lubricant. The magnetic layer 13 may further include an additive suchas conductive particles, an abrasive, or a rust inhibitor as necessary.

The magnetic layer 13 has a surface 13S having a large number ofrecesses 13A. The large number of recesses 13A store a lubricant. Thelarge number of recesses 13A preferably extend in a directionperpendicular to the surface of the magnetic layer 13. This is because aproperty of supplying the lubricant to the surface 13S of the magneticlayer 13 can be improved. Note that some of the large number of recesses13A may extend in the perpendicular direction. Furthermore, on thesurface 13 S of the magnetic layer 13, the recesses 13A having a depthof 20% or more of the average thickness of the magnetic layer 13 areformed at a ratio of, for example, 10 or more and 200 or less,preferably 10 or more and 200 or less, more preferably 20 or more and200 or less per 1600 μm².

The surface density of the recesses 13A on the surface 13S of themagnetic layer 13 is determined, for example, as follows. The surface13S of the magnetic layer 13 is observed by AFM, and an AFM image of 40μm×40 μm is obtained. As the AFM, Dimension 3100, Nano Scope Mamanufactured by Digital Instruments and an analysis software thereof areused. A cantilever including a silicon single crystal (Note 1) is used.Measurement is performed by tuning at 200 to 400 Hz as a tappingfrequency. Next, the AFM image is divided into 512×512 (=262,144)measurement points. The height Z(i) (i: measurement point number, i=1 to262,144) is measured at each measurement point. The measured heightsZ(i) at the measurement points are simply averaged (arithmeticallyaveraged) to determine an average height (reference plane)Z_(ave)(=Z(1)+Z(2)+ . . . +Z(262,144))/262,144). In this case, as imageprocessing, data that has been subjected to filtering processing byFlatten order 2 and plane fit order 3 XY is used as data.

(Note 1) SPM probe NCH normal type Point Probe L (cantilever length)=125μm manufactured by Nano World

FIG. 2A illustrates an example of the surface 13S of the magnetic layer13 observed in an enlarged manner. In FIG. 2A, the XY plane is adirection in which the surface 13S of the magnetic layer 13 extends, andis a region having a surface area of, for example, 40 μm×40 μm=1600 μm².Furthermore, in FIG. 2A, the Z-axis indicates the depth of the recess13A. By counting the number of recesses 13A having a depth from thereference plane corresponding to 20% or more of the average thickness(for example, 70 nm) of the magnetic layer 13 in a region having asurface area of 40 μm×40 μm=1600 μm², the number is determined. FIG. 2Bschematically illustrates a distribution of the plurality of recesses13A in the region having a surface area of 1600 μm² illustrated in FIG.2A. Specifically, a part of a cross section taken along the cut lineIIB-IIB in FIG. 2A is illustrated. In FIG. 2B, the vertical axiscorresponds to the depth of the recess 13A in the Z axis, andspecifically indicates the ratio [%] of the depth of the recess 13A tothe average thickness (for example, 70 nm) of the magnetic layer 13. Inthe cross section of FIG. 2B, the number of recesses 13A having a depthcorresponding to 20% or more of the average thickness (for example, 70nm) of the magnetic layer 13 is two, that is, recesses 13A-1 and 13A-2.FIG. 2C schematically illustrates a distribution of the plurality ofrecesses 13A in the region having a surface area of 1600 μm² illustratedin FIG. 2A. In the example illustrated in FIG. 2C, the number ofrecesses 13A having a depth corresponding to 20% or more of the averagethickness (for example, 70 nm) of the magnetic layer 13 is 33. Note thatthe recess 13A illustrated in FIG. 2C corresponds to the recess 13Aillustrated in FIG. 2A, and the recess 13A-1 and the recess 13A-2illustrated in FIG. 2C correspond to the recess 13A-1 and the recess13A-2 illustrated in FIG. 2B, respectively. Furthermore, as describedlater, the average thickness of the magnetic layer 13 is determined bythinly processing the magnetic recording medium 10 perpendicularly to amain surface thereof to manufacture a sample piece and observing a crosssection of the test piece with a transmission electron microscope (TEM).

The arithmetic average roughness Ra of the surface 13S of the magneticlayer 13 is 2.5 nm or less, preferably 2.2 nm or less, and morepreferably 1.9 nm or less. When the arithmetic average roughness Ra is2.5 nm or less, excellent electromagnetic conversion characteristics canbe obtained. A lower limit value of the arithmetic average roughness Raof the surface 13S of the magnetic layer 13 is preferably 1.0 nm ormore, more preferably 1.2 nm or more, and still more preferably 1.4 nmor more. When the lower limit value of the arithmetic average roughnessRa of the surface 13S of the magnetic layer 13 is 1.0 nm or more, adecrease in traveling performance due to an increase in friction can besuppressed.

The arithmetic average roughness Ra of the surface 13S is determined asfollows. First, a surface of the magnetic layer 13 is observed with anatomic force microscope (AFM) to obtain an AFM image of 40 μm×40 μm. Asthe AFM, Nano Scope Ma D3100 manufactured by Digital Instruments isused. A cantilever including a silicon single crystal is used.Measurement is performed by tuning at 200 Hz to 400 Hz as a tappingfrequency. As the cantilever, for example, “SPM probe NCH normal typePoint Probe L (cantilever length)=125 μm” manufactured by Nano World canbe used. Next, an AFM image is divided into 512×512 (=262,144)measurement points. The height Z(i) (i: measurement point number, i=1 to262,144) is measured at each measurement point. The measured heightsZ(i) at the measurement points are simply averaged (arithmeticallyaveraged) to determine average height (average plane) Zave (=Z(1)+Z(2)+. . . +Z(262,144))/262,144). Subsequently, a deviation Z″(i)(=|Z(i)−Zave|) from an average center line at each measurement point isdetermined, and the arithmetic average roughness Ra [nm](=(Z″(1)+Z″(2)+. . . +Z″(262,144))/262,144) is calculated. In this case, as imageprocessing, data that has been subjected to filtering processing byFlatten order 2 and plane fit order 3 XY is used as data.

A lower limit value of the BET specific surface area of the entiremagnetic recording medium 10 in a state where the lubricant has beenremoved from the magnetic recording medium and the magnetic recordingmedium has been dried is 3.5 m²/g or more, preferably 4 m²/g or more,more preferably 4.5 m²/g or more, and still more preferably 5 m²/g ormore. When the lower limit value of the BET specific surface area is 3.5m²/g or more, even after recording or reproduction is performedrepeatedly (that is, even after the magnetic recording medium 10repeatedly travels while a magnetic head is in contact with a surface ofthe magnetic recording medium 10), it is possible to suppress a decreasein the amount of a lubricant supplied to a space between a surface ofthe magnetic layer 13 and the magnetic head. Therefore, an increase inthe coefficient of dynamic friction can be suppressed.

An upper limit value of the BET specific surface area of the entiremagnetic recording medium 10 in a state where the lubricant has beenremoved from the magnetic recording medium 10 and the magnetic recordingmedium 10 has been dried is preferably 7 m²/g or less, more preferably 6m²/g or less, and still more preferably 5.5 m²/g or less. When the upperlimit value of the BET specific surface area is 7 m²/g or less, alubricant can be sufficiently supplied without being depleted even aftertraveling many times. Therefore, an increase in the coefficient ofdynamic friction can be suppressed.

The magnetic recording medium 10 in a state where the lubricant has beenremoved from the magnetic recording medium 10 and the magnetic recordingmedium 10 has been dried here refers to the magnetic recording medium 10obtained by immersing the magnetic recording medium 10 in hexane at roomtemperature for 24 hours, then taking the magnetic recording medium 10out of hexane, and naturally drying the magnetic recording medium 10.

An average pore diameter of the entire magnetic recording medium 10determined by a BJH method is 6 nm or more and 12 nm or less, preferably7 nm or more and 11 nm or less, and more preferably 7.5 nm or more and11 nm or less. When the average pore diameter is 6 nm or more and 12 nmor less, the above-described effect of suppressing the increase in thecoefficient of dynamic friction can be further improved.

The BET specific surface area and a pore distribution (pore volume andpore diameter of maximum pore volume at the time of desorption) aredetermined as follows. First, the magnetic recording medium 10 having asize about 10% larger than the area 0.1265 m² is immersed in hexane(amount in which the tape can be sufficiently immersed, for example, 150mL) for 24 hours, then naturally dried, and cut out so as to have anarea of 0.1265 m² (for example, 50 cm at each end of the dried tape iscut off to prepare a tape having a width of 10 m) to manufacture ameasurement sample. Next, using a specific surface area/poredistribution measuring device, a pore distribution (pore volume andaverage pore diameter) is determined by a BJH method. A measuring deviceand measuring conditions are indicated below. In this way, the averagediameter of the pores is measured.

Measurement environment: room temperature

Measuring device: 3 FLEX manufactured by Micromeritics Instrument Corp.

Measurement adsorbate: N2 gas

Measurement pressure range (P/P0): 0 to 0.995

For the measurement pressure range, the pressure is changed asillustrated in Table 1 below. The pressure values in Table 1 below arerelative pressures P/P0. In Table below, for example, in step 1, thepressure is changed so as to change by 0.001 per 10 seconds from astarting pressure 0.000 to an ultimate pressure 0.010. When the pressurereaches the ultimate pressure, pressure change in the next step isperformed. This also applies to steps 2 to 10. However, in each step, ina case where the pressure has not reached equilibrium, the device waitsfor the pressure to reach equilibrium and then proceeds to the nextstep.

TABLE 1 Starting Ultimate Step pressure Pressure change pressure 1 0.0000.001/10 sec  0.010 2 0.010 0.02/10 sec 0.100 3 0.100 0.05/10 sec 0.6004 0.600 0.05/10 sec 0.950 5 0.950 0.05/10 sec 0.990 6 0.990 0.05/10 sec0.995 7 0.995 0.01/10 sec 0.990 8 0.990 0.01/10 sec 0.950 9 0.9500.05/10 sec 0.600 10 0.600 0.05/10 sec 0.300

For example, as illustrated in FIG. 3A, the magnetic layer 13 preferablyhas a plurality of servo bands SB and a plurality of data bands DB inadvance. FIG. 3A is a schematic explanatory diagram illustrating alayout of the data bands DB and the servo bands SB in the magneticrecording medium 10, and illustrates a layout in a plane orthogonal to alamination direction in the magnetic recording medium 10 having alaminated structure. As illustrated in FIG. 3A, the plurality of servobands SB is disposed at equal intervals in a width direction of themagnetic recording medium 10. The width direction of the magneticrecording medium 10 is a direction orthogonal to both a longitudinaldirection of the magnetic recording medium 10 and the laminationdirection thereof. A data band DB is disposed between adjacent servobands SB in the width direction. In the servo band SB, a servo signalfor performing tracking control of a magnetic head is written inadvance. User data is recorded in the data band DB.

An upper limit value of a ratio R_(S)(=(S_(SB)/S)×100) of a total areaS_(SB) of the servo bands SB with respect to an area S of the surface13S of the magnetic layer 13 is preferably 4.0% or less, more preferably3.0% or less, and still more preferably 2.0% or less from a viewpoint ofsecuring a high recording capacity. Meanwhile, a lower limit value ofthe ratio R_(S) of the total area S_(SB) of the servo bands SB withrespect to the area S of the surface of the magnetic layer 13 ispreferably 0.8% or more from a viewpoint of securing five or more servotracks.

The ratio R_(S) of the total area S_(SB) of the servo bands SB withrespect to the area S of the surface of the magnetic layer 13 can bemeasured, for example, by developing the magnetic recording medium 10using a ferricolloid developer (Sigmarker Q manufactured by SigmaHi-Chemical Inc.) and then observing the developed magnetic recordingmedium 10 with an optical microscope. The servo band width W_(SB) andthe number of servo bands SB are measured from the observation image ofthe optical microscope. Next, the ratio R_(S) is determined from thefollowing formula.

Ratio R _(S)[%]=(((servo bandwidth W _(SB))×(number of servobands))/(width of magnetic recording medium 10))×100

The number of servo bands SB is preferably 5 or more, and morepreferably 5+4 n (in which n is a positive integer) or more. When thenumber of servo bands SB is 5 or more, an influence on a servo signaldue to a dimensional change of the magnetic recording medium 10 in awidth direction thereof can be suppressed, and stablerecording/reproducing characteristics with less off-track can besecured.

An upper limit value of the servo bandwidth W_(SB) is preferably 95 μmor less, more preferably 60 μm or less, and still more preferably 30 μmor less from a viewpoint of securing a high recording capacity. A lowerlimit value of the servo bandwidth W_(SB) is preferably 10 μm or morefrom a viewpoint of manufacturing a recording head. The width of theservo bandwidth W_(SB) can be determined as follows. First, the magneticrecording medium 10 is developed using a ferricolloid developer(Sigmarker Q manufactured by Sigma Hi-Chemical Inc.). Next, thedeveloped magnetic recording medium 10 is observed with an opticalmicroscope, and the width of the servo bandwidth W_(SB) can be therebymeasured.

As illustrated in FIG. 3B, the magnetic layer 13 can form a plurality ofdata tracks Tk in a data band DB. In this case, an upper limit value ofthe data track width W_(Tk) is preferably 2.0 μm or less, morepreferably 1.5 μm or less, and still more preferably 1.0 μm or less froma viewpoint of securing a high recording capacity. A lower limit valueof the data track width Wm is preferably 0.02 μm or more from aviewpoint of a magnetic particle size.

The magnetic layer 13 can record data such that the minimum value of adistance L between magnetization inversions is preferably 48 nm or less,more preferably 44 nm or less, and still more preferably 40 nm or lessfrom a viewpoint of securing a high recording capacity. The lower limitvalue of the minimum value of the distance L between magnetizationinversions is considered from a viewpoint of a magnetic particle size.

An upper limit value of the average thickness of the magnetic layer 13is preferably 90 nm or less, particularly preferably 80 nm or less, morepreferably 70 nm or less, and still more preferably 50 nm or less. Whenthe upper limit value of the average thickness of the magnetic layer 13is 90 nm or less, in a case where a ring type head is used as arecording head, magnetization can be recorded uniformly in the thicknessdirection of the magnetic layer 13, and therefore electromagneticconversion characteristics can be improved.

A lower limit value of the average thickness of the magnetic layer 13 ispreferably 35 nm or more. When the upper limit value of the averagethickness of the magnetic layer 13 is 35 nm or more, output can besecured in a case where an MR type head is used as a reproducing head,and therefore electromagnetic conversion characteristics can beimproved.

The average thickness of the magnetic layer 13 can be determined asfollows. First, a carbon film is formed on the surface 13S of themagnetic layer 13 of the magnetic recording medium 10 and on a surface14S of the back layer 14 thereof by a vapor deposition method.Thereafter, a tungsten thin film is further formed on the carbon filmcovering the surface 13S of the magnetic layer 13 by a vapor depositionmethod. The carbon film and tungsten film protect a sample in a thinningprocess described later.

Next, the magnetic recording medium 10 is processed to be thinned by afocused ion beam (FIB) method and the like. In a case where the FIBmethod is used, as a pretreatment for observing a TEM image of a crosssection described later, a carbon film and a tungsten thin film areformed as protective films. The carbon film is formed on the magneticlayer side surface of the magnetic recording medium 10 and the backlayer side surface thereof by a vapor deposition method, and thetungsten thin film is further formed on the magnetic layer side surfaceby a vapor deposition method or a sputtering method. The thinning isperformed in a length direction (longitudinal direction) of the magneticrecording medium 10. That is, by the thinning, a cross section parallelto both the longitudinal direction of the magnetic recording medium 10and the thickness direction thereof is formed. The cross section of theobtained thinned sample is observed with a transmission electronmicroscope (TEM) under the following conditions to obtain a TEM image.Note that the magnification and the acceleration voltage may beappropriately adjusted depending on the type of a device.

Device: TEM (H9000NAR manufactured by Hitachi, Ltd.)

Acceleration voltage: 300 kV

Magnification: 100,000 times

Next, using the obtained TEM image, the thickness of the magnetic layer13 is measured at ten or more points in the longitudinal direction ofthe magnetic recording medium 10. An average value obtained by simplyaveraging (arithmetically averaging) the obtained measured values istaken as the average thickness of the magnetic layer 13. Note that thepositions where the measurement is performed are randomly selected froma test piece.

(Magnetic Powder)

The magnetic powder includes, for example, powder of a nanoparticleincluding ε iron oxide (hereinafter referred to as “c iron oxideparticle”). The ε iron oxide particle can obtain high coercive forceeven if the ε iron oxide particle is a fine particle. ε iron oxideincluded in the ε iron oxide particle is preferably crystal-orientedpreferentially in a thickness direction (perpendicular direction) of themagnetic recording medium 10.

FIG. 4 is a cross-sectional view schematically illustrating an exampleof a cross-sectional structure of an ε iron oxide particle 20 includedin the magnetic layer 13. As illustrated in FIG. 4 , the ε iron oxideparticle 20 has a spherical shape or a substantially spherical shape, orhas a cubic shape or a substantially cubic shape. Since the ε iron oxideparticle 20 has the shape as described above, in a case where the ε ironoxide particle 20 is used as a magnetic particle, a contact area betweenthe particles in the thickness direction of the magnetic recordingmedium 10 can be reduced, and aggregation of the particles can besuppressed as compared to a case where a hexagonal plate-shaped bariumferrite particle is used as the magnetic particle. Therefore,dispersibility of the magnetic powder can be enhanced, and a bettersignal-to-noise ratio (SNR) can be obtained.

The ε iron oxide particle 20 has, for example, a core-shell typestructure. Specifically, as illustrated in FIG. 4 , the ε iron oxideparticle 20 has a core portion 21 and a two-layered shell portion 22disposed around the core portion 21. The two-layered shell portion 22includes a first shell portion 22 a disposed on the core portion 21 anda second shell portion 22 b disposed on the first shell portion 22 a.

The core portion 21 in the ε iron oxide particle 20 includes ε ironoxide. ε iron oxide included in the core portion 21 preferably includesan ε—Fe₂O₃ crystal as a main phase, and more preferably includes ε—Fe₂O₃as a single phase.

The first shell portion 22 a covers at least a part of the periphery ofthe core portion 21. Specifically, the first shell portion 22 a maypartially cover the periphery of the core portion 21 or may cover theentire periphery of the core portion 21. The first shell portion 22 apreferably covers the entire surface of the core portion 21 from aviewpoint of making exchange coupling between the core portion 21 andthe first shell portion 22 a sufficient and improving magneticcharacteristics.

The first shell portion 22 a is a so-called soft magnetic layer, andincludes, for example, a soft magnetic material such as α—Fe, a Ni—Fealloy, or a Fe—Si—Al alloy. α—Fe may be obtained by reducing ε ironoxide included in the core portion 21.

The second shell portion 22 b is an oxide film as an antioxidant layer.The second shell portion 22 b includes a iron oxide, aluminum oxide, orsilicon oxide. α-iron oxide includes, for example, at least one ironoxide of Fe₃O₄, Fe₂O₃, and FeO. In a case where the first shell portion22 a includes α—Fe (soft magnetic material), α-iron oxide may beobtained by oxidizing a-Fe included in the first shell portion 22 a.

By inclusion of the first shell portion 22 a in the ε iron oxideparticle 20 as described above, a coercive force Hc of the entire ironoxide particle (core-shell particle) 20 can be adjusted to a coerciveforce Hc suitable for recording while a coercive force Hc of the coreportion 21 alone is maintained at a large value in order to securethermal stability. Furthermore, by inclusion of the second shell portion22 b in the ε iron oxide particle 20 as described above, it is possibleto suppress deterioration of the characteristics of the iron oxideparticle 20 due to generation of a rust and the like on a surface of theparticle by exposure of the ε iron oxide particle 20 to the air during astep of manufacturing the magnetic recording medium 10 and before thestep. Therefore, characteristic deterioration of the magnetic recordingmedium 10 can be suppressed by covering the first shell portion 22 awith the second shell portion 22 b.

The average particle size (average maximum particle size) of themagnetic powder is preferably 25 nm or less, more preferably 8 nm ormore and 22 nm or less, and still more preferably 12 nm or more and 22nm or less. In the magnetic recording medium 10, an area having a halfsize of a recording wavelength is an actual magnetization area.Therefore, by setting the average particle size of the magnetic powderto a half or less of the shortest recording wavelength, a good S/N canbe obtained. Therefore, when the average particle size of the magneticpowder is 22 nm or less, in the magnetic recording medium 10 having ahigh recording density (for example, the magnetic recording medium 10that can record a signal at the shortest recording wavelength of 50 nmor less), good electromagnetic conversion characteristics (for example,SNR) can be obtained. Meanwhile, when the average particle size of themagnetic powder is 8 nm or more, dispersibility of the magnetic powderis further improved, and better electromagnetic conversioncharacteristics (for example, SNR) can be obtained.

The magnetic powder has an average aspect ratio of preferably 1 or moreand 3.0 or less, more preferably 1 or more and 2.8 or less, still morepreferably 1 or more and 1.8 or less. When the average aspect ratio ofthe magnetic powder is within a range of 1 or more and 3.0 or less,aggregation of the magnetic powder can be suppressed, and resistanceapplied to the magnetic powder can be suppressed when the magneticpowder is vertically oriented in a step of forming the magnetic layer13. Therefore, perpendicular orientation of the magnetic powder can beimproved.

The average particle size and the average aspect ratio of the magneticpowder described above can be determined as follows. First, the magneticrecording medium 10 to be measured is processed to be thinned by afocused ion beam (FIB) method and the like. Thinning is performed in thelength direction (longitudinal direction) of the magnetic tape. That is,this thinning forms a cross section parallel to both the longitudinaldirection of the magnetic recording medium 10 and the thicknessdirection thereof. Cross-sectional observation is performed for theobtained thin sample such that the entire magnetic layer 13 is includedwith respect to the thickness direction of the magnetic layer 13 using atransmission electron microscope (H-9500 manufactured by HitachiHigh-Technologies) with an acceleration voltage of 200 kV and an overallmagnification of 500,000 times, and a TEM photograph is imaged. Next, 50particles are randomly selected from the imaged TEM photograph, and along axis length DL and a short axis length DS of each of the particlesare measured. Here, the long axis length DL means the largest distanceamong distances between two parallel lines drawn from all angles so asto come into contact with an outline of each of the particles (so-calledmaximum Feret diameter). Meanwhile, the short axis length DS means thelargest length among the lengths of a particle in a direction orthogonalto the long axis length DL of the particle.

Subsequently, the measured long axis lengths DL of the 50 particles aresimply averaged (arithmetically averaged) to determine an average longaxis length DLave. The average long axis length DLave determined in thismanner is taken as an average particle size of the magnetic powder.Furthermore, the measured short axis lengths DS of the 50 particles aresimply averaged (arithmetically averaged) to determine an average shortaxis length DSave. Then, an average aspect ratio (DLave/DSave) of theparticle is determined from the average long axis length DLave and theaverage short axis length DSave.

The average particle volume of the magnetic powder is preferably 5500nm³ or less, more preferably 270 nm³ or more and 5500 nm³ or less, andstill more preferably 900 nm³ or more and 5500 nm³ or less. When theaverage particle volume of the magnetic powder is 5500 nm³ or less, asimilar effect to that in a case where the average particle size of themagnetic powder is 22 nm or less can be obtained. Meanwhile, when theaverage particle volume of the magnetic powder is 270 nm³ or more, asimilar effect to a case where the average particle size of the magneticpowder is 8 nm or more can be obtained.

In a case where the ε iron oxide particle 20 has a spherical shape or asubstantially spherical shape, the average particle volume of themagnetic powder is determined as follows. First, an average long axislength DLave is determined in a similar manner to the above-describedmethod for calculating the average particle size of the magnetic powder.Next, an average volume V of the magnetic powder is determined by thefollowing formula.V=(π/6)×(DLave)³

(Binder)

As the binder, a resin having a structure in which a crosslinkingreaction is imparted to a polyurethane-based resin, a vinylchloride-based resin, and the like is preferable. However, the binder isnot limited to these resins, and other resins may be blendedappropriately according to physical properties and the like required forthe magnetic recording medium 10. Usually, a resin to be blended is notparticularly limited as long as being generally used in the applicationtype magnetic recording medium 10.

Examples of the resin to be blended include polyvinyl chloride,polyvinyl acetate, a vinyl chloride-vinyl acetate copolymer, a vinylchloride-vinylidene chloride copolymer, a vinyl chloride-acrylonitrilecopolymer, an acrylate-acrylonitrile copolymer, an acrylate-vinylchloride-vinylidene chloride copolymer, a vinyl chloride-acrylonitrilecopolymer, an acrylate-acrylonitrile copolymer, an acrylate-vinylidenechloride copolymer, a methacrylate-vinylidene chloride copolymer, amethacrylate-vinyl chloride copolymer, a methacryate-ethylene copolymer,polyvinyl fluoride, a vinylidene chloride-acrylonitrile copolymer, anacrylonitrile-butadiene copolymer, a polyamide resin, polyvinyl butyral,a cellulose derivative (cellulose acetate butyrate, cellulose diacetate,cellulose triacetate, cellulose propionate, and nitrocellulose), astyrene-butadiene copolymer, a polyester resin, an amino resin, asynthetic rubber, and the like.

Furthermore, examples of a thermosetting resin or a reactive resininclude a phenol resin, an epoxy resin, a urea resin, a melamine resin,an alkyd resin, a silicone resin, a polyamine resin, and a ureaformaldehyde resin.

Furthermore, in order to improve dispersibility of the magnetic powder,a polar functional group such as —SO₃M, —OSO₃M, —COOM, or P═O(OM)₂ maybe introduced into each of the above-described binders. Here, in thechemical formulas described above, M represents a hydrogen atom or analkali metal such as lithium, potassium, or sodium.

Moreover, examples of the polar functional group include a side chaintype group having a terminal group of —NR1R2 or —NR1R2R3⁺X⁻, and a mainchain type group of >NR1R2⁺X⁻. Here, in the formulas described above,R1, R2, and R3 each represent a hydrogen atom or a hydrocarbon group,and X⁻ represents an ion of a halogen element such as fluorine,chlorine, bromine, or iodine, or an inorganic or organic ion.Furthermore, examples of the polar functional group include —OH, —SH,—CN, and an epoxy group.

(Lubricant)

The lubricant included in the magnetic layer 13 includes, for example, afatty acid and a fatty acid ester. The fatty acid included in thelubricant preferably includes, for example, at least one of a compoundrepresented by the following general formula <1> and a compoundrepresented by the general formula <2>. Furthermore, the fatty acidester included in the lubricant preferably includes at least one of acompound represented by the following general formula <3> and a compoundrepresented by the general formula <4>. By inclusion of two compounds ofa compound represented by general formula <1> and a compound representedby general formula <3>, inclusion of two compounds of a compoundrepresented by general formula <2> and a compound represented by generalformula <3>, inclusion of two compounds of a compound represented bygeneral formula <1> and a compound represented by general formula <4>,inclusion of two compounds of a compound represented by general formula<2> and a compound represented by general formula <4>, inclusion ofthree compounds of a compound represented by general formula <1>, acompound represented by general formula <2>, and a compound representedby general formula <3>, inclusion of three compounds of a compoundrepresented by general formula <1>, a compound represented by generalformula <2>, and a compound represented by general formula <4>,inclusion of three compounds of a compound represented by generalformula <1>, a compound represented by general formula <3>, and acompound represented by general formula <4>, inclusion of threecompounds of a compound represented by general formula <2>, a compoundrepresented by general formula <3>, and a compound represented bygeneral formula <4>, or inclusion of four compounds of a compoundrepresented by general formula <1>, a compound represented by generalformula <2>, a compound represented by general formula <3>, and acompound represented by general formula <4> in the lubricant, anincrease in the coefficient of dynamic friction due to repeatedrecording or reproduction in the magnetic recording medium 10 can besuppressed. As a result, traveling performance of the magnetic recordingmedium 10 can be further improved.CH₃(CH₂)_(k)COOH  <1>

(Provided that in general formula <1>, k is an integer selected from arange of 14 or more and 22 or less, more preferably a range of 14 ormore and 18 or less.)CH₃(CH₂)_(n)CH═CH(CH₂)_(m)COOH  <2>

(Provided that in general formula <2>, the sum of n and m is an integerselected from a range of 12 or more and 20 or less, more preferably arange of 14 or more and 18 or less.)CH₃(CH₂)_(p)COO(CH₂)_(q)CH₃  <3>

(Provided that in general formula <3>, p is an integer selected from arange of 14 or more and 22 or less, more preferably a range of 14 ormore and 18 or less, and q is an integer selected from a range of 2 ormore and 5 or less, more preferably a range of 2 or more and 4 or less.)CH₃(CH₂)_(p)COO—(CH₂)_(q)CH(CH₃)₂  <4>

(Provided that in the general formula <2>, p is an integer selected froma range of 14 or more and 22 or less, and q is an integer selected froma range of 1 or more and 3 or less.)

(Additive)

As nonmagnetic reinforcing particles, the magnetic layer 13 may furtherinclude aluminum oxide (a, (3, or y alumina), chromium oxide, siliconoxide, diamond, garnet, emery, boron nitride, titanium carbide, siliconcarbide, titanium carbide, titanium oxide (rutile type or anatase typetitanium oxide), and the like.

(Base Layer 12)

The base layer 12 is a nonmagnetic layer including nonmagnetic powderand a binder. The base layer 12 may further include at least oneadditive selected from a lubricant, conductive particles, a curingagent, a rust inhibitor, and the like as necessary. Furthermore, thebase layer 12 may have a multi-layered structure formed by laminating aplurality of layers. An average thickness of the base layer 12 ispreferably 0.5 μm or more and 0.9 μm or less, and more preferably 0.6 μmor more and 0.7 μm or less. By reducing the average thickness of thebase layer 12 to 0.9 μm or less, the Young's modulus of the entiremagnetic recording medium 10 is more effectively reduced than that in acase where the thickness of the substrate 11 is reduced. For thisreason, tension control with respect to the magnetic recording medium 10is easy. Furthermore, by setting the average thickness of the base layer12 to 0.5 μm or more, adhesive force between the substrate 11 and thebase layer 12 is secured. In addition, variations in the thickness ofthe base layer 12 can be suppressed, and an increase in the roughness ofthe surface 13S of the magnetic layer 13 can be prevented.

Note that the average thickness of the base layer 12 is obtained asfollows, for example. First, the magnetic recording medium 10 having awidth of ½ inches is prepared and cut into a length of 250 mm tomanufacture a sample. Subsequently, in the magnetic recording medium 10as the sample, the base layer 12 and the magnetic layer 13 are peeledoff from the substrate 11. Next, using a laser hologauge (LGH-110C)manufactured by Mitutoyo Corporation as a measuring device, thethickness of a laminate of the base layer 12 and the magnetic layer 13peeled off from the substrate 11 is measured at five or more points.Thereafter, the measured values are simply averaged (arithmeticallyaveraged) to calculate the average thickness of the laminate of the baselayer 12 and the magnetic layer 13. Note that the measurement points arerandomly selected from the sample. Finally, the average thickness of thebase layer 12 is determined by subtracting the average thickness of themagnetic layer 13 measured using TEM as described above from the averagethickness of the laminate.

The base layer 12 may have pores, that is, the base layer 12 may have alarge number of pores. The pores of the base layer 12 may be formed, forexample, along with formation of pores (recesses 13A) in the magneticlayer 13, and in particular, are formed by pressing a large number ofprotrusions formed on the surface 14S of the back layer 14 of themagnetic recording medium 10 against the magnetic layer side surface.That is, by forming a recess corresponding to the shape of a protrusionon the surface 13S of the magnetic layer 13, pores can be formed in eachof the magnetic layer 13 and the base layer 12. Furthermore, pores maybe formed as a solvent volatilizes in a step of drying a magnetic layerforming coating material. Furthermore, when the magnetic layer formingcoating material is applied to a surface of the base layer 12 in orderto form the magnetic layer 13, a solvent in the magnetic layer formingcoating material passes through the pores of the base layer 12 formedwhen the lower layer is applied and dried, and can permeate the baselayer 12. Thereafter, when the solvent that has permeated the base layer12 volatilizes in a step of drying the magnetic layer 13, the solventthat has permeated the base layer 12 moves from the base layer 12 to thesurface 13S of the magnetic layer 13, thereby pores may be formed. Thepores formed in this way can communicate, for example, the magneticlayer 13 with the base layer 12. The average diameter of the pores canbe adjusted by changing the solid content of the magnetic layer formingcoating material or the type of a solvent thereof and/or dryingconditions of the magnetic layer forming coating material. By formingpores in both the magnetic layer 13 and the base layer 12, aparticularly suitable amount of lubricant for good traveling stabilityappears on the magnetic layer side surface, and an increase in thecoefficient of dynamic friction due to repeated recording orreproduction can be further suppressed.

Holes of the base layer 12 are preferably connected to the recesses 13Aof the magnetic layer 13 from a viewpoint of suppressing a decrease inthe coefficient of dynamic friction after repeated recording orreproduction. Here, the state where the holes of the base layer 12 areconnected to the recesses 13A of the magnetic layer 13 includes a statewhere some of the large number of holes of the base layer 12 areconnected to some of the large number of recesses 13A of the magneticlayer 13.

The large number of recesses 13A preferably include those extending in adirection perpendicular to the surface 13S of the magnetic layer 13 froma viewpoint of improving a property of supplying the lubricant to thesurface 13S of the magnetic layer 13. Furthermore, the holes of the baselayer 12 extending in a direction perpendicular to the surface 13S ofthe magnetic layer 13 are preferably connected to the recesses 13A ofthe magnetic layer 13 extending in the direction perpendicular to thesurface 13S of the magnetic layer 13 from a viewpoint of improving aproperty of supplying the lubricant to the surface 13S of the magneticlayer 13.

(Nonmagnetic Powder of Base Layer 12)

The nonmagnetic powder includes, for example, at least one of inorganicparticle powder and organic particle powder. Furthermore, thenonmagnetic powder may include carbon powder such as carbon black. Notethat one kind of nonmagnetic powder may be used singly, or two or morekinds of nonmagnetic powder may be used in combination. Examples of theinorganic powder include a metal, a metal oxide, a metal carbonate, ametal sulfate, a metal nitride, a metal carbide, a metal sulfide, andthe like. Examples of the shape of the nonmagnetic powder includevarious shapes such as an acicular shape, a spherical shape, a cubicshape, and a plate shape, but are not limited thereto.

(Binder in Base Layer 12)

The binder in the base layer 12 is similar to that in the magnetic layer13 described above.

(Back Layer 14)

The back layer 14 includes, for example, a binder and nonmagneticpowder. The back layer 14 may further include at least one additiveselected from a lubricant, a curing agent, an antistatic agent, and thelike as necessary. The binder and nonmagnetic powder in the back layer14 are similar to those in the base layer 12 described above.

The nonmagnetic powder in the back layer 14 has an average particle sizeof preferably 10 nm or more and 150 nm or less, more preferably 15 nm ormore and 110 nm or less. The average particle size of the nonmagneticpowder of the back layer 14 is determined in a similar manner to theaverage particle size of the magnetic powder in the magnetic layer 13described above. The nonmagnetic powder may include those having aparticle size distribution of 2 or more.

The kurtosis (Sku) of the surface 14S of the back layer 14 opposite tothe substrate 11 is preferably 2.0 or more and 4.0 or less. This isbecause winding deviation can be effectively suppressed. However, thekurtosis (Sku) of the surface 14S is more preferably 3.0 or more and 4.0or less, and still more preferably 3.5 nm or more and 4.0 or less inorder to improve the effect of preventing winding deviation.

The kurtosis of the surface 14S is measured as follows. First, themagnetic recording medium 10 having a width of 12.65 mm is prepared andcut into a length of 100 mm to manufacture a sample. Next, the sample isplaced on a slide glass with a surface of the sample to be measured (thesurface 14S of the back layer 14) upward, and an end of the sample isfixed with a mending tape. The shape of the surface is measured usingVert Scan (objective lens: 50 times) as a measuring device, and thekurtosis Sku is determined from the formula described in the followingnumerical formula 1 on the basis of the ISO 25178 standard.

-   -   Device: non-contact roughness meter using optical interference

(Non-contact surface/layer cross-sectional shape measurement system VertScan R5500GL-M100-AC manufactured by Ryoka Systems Inc.)

-   -   Objective lens: 50 times    -   CCD: ⅓ lens    -   Measurement area: 640×480 pixels (field of view: about 95 μm×71        μm)    -   Measurement mode: phase    -   Wavelength filter: 520 nm    -   Noise removal filter smoothing 3×3    -   Surface correction: corrected with quadratic polynomial        approximation surface    -   Measurement software: VS-Measure Version 5.5.2    -   Analysis software: VS-viewer Version 5.5.5

As described above, the kurtosis is measured at five or more points inthe longitudinal direction, and then an average of the obtained valuesat five points is taken.

$\begin{matrix}{S_{ku} = {\frac{1}{S_{q}^{4}}\left\lbrack {\frac{1}{A}{\int{\int_{A}{{Z^{4}\left( {x,y} \right)}{dxdy}}}}} \right\rbrack}} & \left\lbrack {{Numerical}{Formula}1} \right\rbrack\end{matrix}$Here, the symbols in the formula represent the following.

$S_{q} = \sqrt{\frac{1}{A}{\int{\int_{A}{{Z^{2}\left( {x,y} \right)}{dxdy}}}}}$A: number of samplesx: horizontal direction of sampley: vertical direction of samplez: height

An upper limit value of the average thickness of the back layer 14 ispreferably 0.6 μm or less, and particularly preferably 0.5 μm or less.When the upper limit value of the average thickness of the back layer 14is 0.6 μm or less, even in a case where the average thickness of themagnetic recording medium 10 is 5.6 μm or less, the thicknesses of thebase layer 12 and the substrate 11 can be kept thick. Therefore,traveling stability of the recording medium 10 in therecording/reproducing device can be maintained. The lower limit value ofthe average thickness of the back layer 14 is not particularly limited,but is, for example, 0.2 μm or more, and particularly preferably 0.3 μmor more.

The average thickness of the back layer 14 is determined as follows.First, the magnetic recording medium 10 having a width of ½ inches isprepared and cut into a length of 250 mm to manufacture a sample. Next,the thickness of the magnetic recording medium 10 as a sample ismeasured at five or more points using a laser hologage (LGH-110C)manufactured by Mitutoyo Corporation as a measuring device, and themeasured values are simply averaged (arithmetically averaged) tocalculate the average thickness t_(T) [μm] of the magnetic recordingmedium 10. Note that the measurement points are randomly selected fromthe sample. Subsequently, the back layer 14 is removed from the magneticrecording medium 10 as a sample with a solvent such as methyl ethylketone (MEK) or dilute hydrochloric acid. Thereafter, using the laserhologauge described above again, the thickness of the sample obtained byremoving the back layer 14 from the magnetic recording medium 10 ismeasured at five or more points, and these measured values are simplyaveraged (arithmetically averaged) to calculate the average thicknesst_(B) [μm] of the magnetic recording medium 10 from which the back layer14 has been removed. Note that the measurement points are randomlyselected from the sample. Finally, the average thickness t_(b)[μm] ofthe back layer 14 is determined by the following formula.t _(b)[μm]=t _(T)[μm]−t _(B)[μm]

As illustrated in FIG. 1 , the back layer 14 has a surface having alarge number of protrusions 14A. The large number of protrusions 14A areused for forming the large number of recesses 13A on the surface 13S ofthe magnetic layer 13 in a state where the magnetic recording medium 10has been wound up in a roll shape. The large number of recesses 13A areformed by, for example, a large number of nonmagnetic particlesprotruding from a surface of the back layer 14.

(Average Thickness of Magnetic Recording Medium 10)

An upper limit value of the average thickness (average total thickness)of the magnetic recording medium 10 is preferably 5.6 μm or less, morepreferably 5.0 μm or less, particularly preferably 4.6 μm or less, andstill more preferably 4.4 μm or less. When the average thickness of themagnetic recording medium 10 is 5.6 μm or less, the recording capacitythat can be recorded in one data cartridge can be increased as comparedto a general magnetic recording medium. A lower limit value of theaverage thickness of the magnetic recording medium 10 is notparticularly limited, but is, for example, 3.5 μm or more.

The average thickness t_(T) of the magnetic recording medium 10 isobtained as follows. First, the magnetic recording medium 10 having awidth of ½ inches is prepared and cut into a length of 250 mm tomanufacture a sample. Next, the thickness of the sample is measured atfive or more points using a laser hologage (LGH-110C) manufactured byMitutoyo Corporation as a measuring device, and the measured values aresimply averaged (arithmetically averaged) to calculate the average valuet_(T) [μm]. Note that the measurement points are randomly selected fromthe sample.

(Coercive Force Hc)

An upper limit value of the coercive force Hc of the magnetic recordingmedium 10 in a longitudinal direction thereof is preferably 2000 Oe orless, more preferably 1900 Oe or less, and still more preferably 1800 Oeor less. When the coercive force Hc2 in the longitudinal direction is2000 Oe or less, magnetization reacts with high sensitivity due to amagnetic field in a perpendicular direction from a recording head.Therefore, a good recording pattern can be formed.

A lower limit value of the coercive force Hc measured in thelongitudinal direction of the magnetic recording medium 10 is preferably1000 Oe or more. When the lower limit value of the coercive force Hc inthe longitudinal direction is 1000 Oe or more, demagnetization due to aleakage magnetic flux from a recording head can be suppressed.

The coercive force Hc described above is determined as follows. Threemagnetic recording media 10 are overlapped and bonded with adouble-sided tape, and then punched with a 0.39 mm punch to manufacturea measurement sample. At this time, marking is performed with anarbitrary ink having no magnetism such that the longitudinal direction(traveling direction) of the magnetic recording medium can berecognized. Then, using a vibrating sample magnetometer (VSM), an M-Hloop of the measurement sample (the entire magnetic recording medium 10)corresponding to the longitudinal direction of the magnetic recordingmedium 10 (traveling direction of the magnetic recording medium 10) ismeasured. Next, the coating film (the base layer 12, the magnetic layer13, the back layer 14, and the like) is wiped off using acetone,ethanol, and the like, leaving only the substrate 11. Then, the threesubstrates 11 thus obtained are overlapped and bonded with adouble-sided tape, and then punched with a 0.39 mm punch to obtain abackground correction sample (hereinafter simply referred to as acorrection sample). Thereafter, an M-H loop of the correction sample(substrate 11) corresponding to the longitudinal direction of thesubstrate 11 (traveling direction of the magnetic recording medium 10)is measured using VSM.

In the measurement of the M-H loop of the measurement sample (the entiremagnetic recording medium 10) and the M-H loop of the correction sample(substrate 11), for example, a favorably sensitive vibrating samplemagnetometer “VSM-P7-15 type” manufactured by Toei Industry Co., Ltd. isused. The measurement conditions are set to measurement mode: full loop,maximum magnetic field: 15 kOe, magnetic field step: 40 bits, timeconstant of locking amp: 0.3 sec, waiting time: 1 sec, and MH averagenumber: 20.

After the two M-H loops are obtained, the M-H loop of the correctionsample (substrate 11) is subtracted from the M-H loop of the measurementsample (the entire magnetic recording medium 10) to perform backgroundcorrection, and an M-H loop after background correction is obtained. Forthe calculation of background correction, a measurement/analysis programattached to “VSMP7-15 type” is used.

The coercive force Hc is determined from the obtained M-H loop afterbackground correction. Note that for this calculation, ameasurement/analysis program attached to “VSM-P7-15” is used. Note thateach of the above measurements of the M-H loops is performed at 25° C.Furthermore, when the M-H loop is measured in the longitudinal directionof the magnetic recording medium 10, “demagnetizing field correction” isnot performed.

(Squareness Ratio)

The magnetic recording medium 10 has a squareness ratio S1 of, forexample, 65% or more, preferably 70% or more, more preferably 75% ormore, still more preferably 80% or more, particularly preferably 85% ormore in a perpendicular direction (thickness direction) of the magneticrecording medium 10. When the squareness ratio S1 is 65% or more,perpendicular orientation of magnetic powder is sufficiently high.Therefore, better SNR can be obtained.

The squareness ratio S1 is determined as follows. Three magneticrecording media 10 are overlapped and bonded with a double-sided tape,and then punched with a φ6.39 mm punch to manufacture a measurementsample. At this time, marking is performed with an arbitrary ink havingno magnetism such that the longitudinal direction (traveling direction)of the magnetic recording medium can be recognized. Then, using avibrating sample magnetometer (VSM), an M-H loop of the measurementsample (the entire magnetic recording medium 10) corresponding to thelongitudinal direction of the magnetic recording medium 10 (travelingdirection of the magnetic recording medium 10) is measured. Next, thecoating film (the base layer 12, the magnetic layer 13, the back layer14, and the like) is wiped off using acetone, ethanol, and the like,leaving only the substrate 11. Then, the three substrates 11 thusobtained are overlapped and bonded with a double-sided tape, and thenpunched with a 0.39 mm punch to obtain a background correction sample(hereinafter simply referred to as a correction sample). Thereafter, anM-H loop of the correction sample (substrate 11) corresponding to thelongitudinal direction of the substrate 11 (traveling direction of themagnetic recording medium 10) is measured using VSM.

In the measurement of the M-H loop of the measurement sample (the entiremagnetic recording medium 10) and the M-H loop of the correction sample(substrate 11), for example, a favorably sensitive vibrating samplemagnetometer “VSM-P7-15 type” manufactured by Toei Industry Co., Ltd. isused. The measurement conditions are set to measurement mode: full loop,maximum magnetic field: 15 kOe, magnetic field step: 40 bits, timeconstant of locking amp: 0.3 sec, waiting time: 1 sec, and MH averagenumber: 20.

After the two M-H loops are obtained, the M-H loop of the correctionsample (substrate 11) is subtracted from the M-H loop of the measurementsample (the entire magnetic recording medium 10) to perform backgroundcorrection, and an M-H loop after background correction is obtained. Forthe calculation of background correction, a measurement/analysis programattached to “VSMP7-15 type” is used.

The squareness ratio S1(%) is calculated by putting saturationmagnetization Ms (emu) and residual magnetization Mr (emu) of theobtained M-H loop after background correction into the followingformula.Squareness ratio S1(%)=(Mr/Ms)×100

Note that each of the above measurements of the M-H loops is performedat 25° C. Furthermore, when the M-H loop is measured in theperpendicular direction of the magnetic recording medium 10,“demagnetizing field correction” is not performed.

The magnetic recording medium 10 has a squareness ratio S2 of preferably35% or less, more preferably 30% or less, still more preferably 25% orless, particularly preferably 20% or less, most preferably 15% or lessin the longitudinal direction (traveling direction) of the magneticrecording medium 10. When the squareness ratio S2 is 35% or less,perpendicular orientation of magnetic powder is sufficiently high.Therefore, better SNR can be obtained.

The squareness ratio S2 is determined in a similar manner to thesquareness ratio S1 except that the M-H loop is measured in thelongitudinal direction (traveling direction) of the magnetic recordingmedium 10 and the substrate 11.

(SFD)

In a switching field distribution (SFD) curve of the magnetic recordingmedium 10, a peak ratio X/Y between a height X of a main peak and aheight Y of a sub-peak near the magnetic field zero is preferably 3.0 ormore, more preferably 5.0 or more, still more preferably 7.0 or more,particularly preferably 10.0 or more, and most preferably 20.0 or more(refer to FIG. 5 ). When the peak ratio X/Y is 3.0 or more, it ispossible to suppress inclusion of a large amount of low coercive forcecomponents unique to ε iron oxide (for example, soft magnetic particles,superparamagnetic particles, or the like) in magnetic powder in additionto the ε iron oxide particle 20 contributing to actual recording.Therefore, it is possible to suppress deterioration of a magnetizationsignal recorded in an adjacent track due to a leakage magnetic fieldfrom a recording head. Therefore, better SNR can be obtained. An upperlimit value of the peak ratio X/Y is not particularly limited, but isfor example, 100 or less.

The peak ratio X/Y described above is determined as follows. First, in asimilar manner to the above method for measuring a coercive force Hc, anM-H loop after background correction is obtained. Next, an SFD curve iscalculated from the obtained M-H loop. For calculating the SFD curve, aprogram attached to a measuring machine may be used, or another programmay be used. By taking an absolute value of a point where the calculatedSFD curve crosses the Y axis (dM/dH) as “Y” and taking the height of amain peak seen near a coercive force Hc in the M-H loop as “X”, the peakratio X/Y is calculated. Note that the M-H loop is measured at 25° C. ina similar manner to the above method for measuring a coercive force Hc.Furthermore, when the M-H loop is measured in the thickness direction(perpendicular direction) of the magnetic recording medium 10,“demagnetizing field correction” is not performed. Furthermore,according to the sensitivity of VSM used, a plurality of samples to bemeasured may be stacked on each other to measure the M-H loop.

(Activation Volume Vact)

An activation volume Vact is preferably 8000 nm³ or less, morepreferably 6000 nm³ or less, still more preferably 5000 nm³ or less,particularly preferably 4000 nm³ or less, and most preferably 3000 nm³or less. When the activation volume Vact is 8000 nm³ or less, adispersed state of magnetic powder is good. Therefore, a bit inversionregion can be made steep, and it is possible to suppress deteriorationof a magnetization signal recorded in an adjacent track due to a leakagemagnetic field from a recording head. Therefore, a better SNR can beobtained.

The activation volume Vact described above is determined by thefollowing formula derived by Street & Woolley.Vact(nm³)=kB×T×Xirr/(μ0×Ms×S)

(In which kB: Boltzmann's constant (1.38×10′⁻²³ J/K), T: temperature(K), Xirr: irreversible susceptibility, μ0: vacuum permeability, S:magnetic viscosity coefficient, Ms: saturation magnetization (emu/cm³))

The irreversible susceptibility Xirr, the saturation magnetization Ms,and the magnetic viscosity coefficient S to be put in the above formulaare determined using VSM as follows. A measurement sample used for VSMis manufactured by punching out a product obtained by overlapping threemagnetic recording media 10 with a double-sided tape with a 0.39 mmpunch. At this time, marking is performed with an arbitrary ink havingno magnetism such that the longitudinal direction (traveling direction)of the magnetic recording medium 10 can be recognized. Note that ameasurement direction using VSM is the thickness direction(perpendicular direction) of the magnetic recording medium 10.Furthermore, the measurement using VSM is performed at 25° C. for ameasurement sample cut out from the long magnetic recording medium 10.Furthermore, when the M-H loop is measured in the thickness direction(perpendicular direction) of the magnetic recording medium 10,“demagnetizing field correction” is not performed. Moreover, in themeasurement of the M-H loop of the measurement sample (the entiremagnetic recording medium 10) and the M-H loop of the correction sample(substrate 11), a highly sensitive vibrating sample magnetometer“VSM-P7-15 type” manufactured by Toei Industry Co., Ltd. is used. Themeasurement conditions are set to measurement mode: full loop, maximummagnetic field: 15 kOe, magnetic field step: 40 bits, time constant oflocking amp: 0.3 sec, waiting time: 1 sec, and MH average number: 20.

(Irreversible Susceptibility Xirr)

The irreversible susceptibility Xirr is defined as an inclination near aresidual coercive force Hr in the inclination of a residualmagnetization curve (DCD curve). First, a magnetic field of −1193 kA/m(15 kOe) is applied to the entire magnetic recording medium 10, and themagnetic field is returned to zero to obtain a residual magnetizationstate. Thereafter, a magnetic field of about 15.9 kA/m (200 Oe) isapplied in the opposite direction to return the magnetic field to zeroagain, and a residual magnetization amount is measured. Thereafter,similarly, measurement of applying a magnetic field larger than thepreviously applied magnetic field by 15.9 kA/m to return the magneticfield to zero is repeated, and a residual magnetization amount isplotted with respect to an applied magnetic field to form a DCD curve.From the obtained DCD curve, a point where the magnetization amount iszero is taken as a residual coercive force Hr, the DCD curve isdifferentiated, and the inclination of the DCD curve at each magneticfield is determined. In the inclination of this DCD curve, aninclination near the residual coercive force Hr is Xirr.

(Saturation Magnetization Ms)

First, in a similar manner to the above method for measuring a coerciveforce Hc, an M-H loop after background correction is obtained. Next, Ms(emu/cm³) is calculated from a value of saturation magnetization Ms(emu) of the obtained M-H loop and the volume (cm³) of the magneticlayer 13 in the measurement sample. Note that the volume of the magneticlayer 13 is determined by multiplying the area of the measurement sampleby an average thickness of the magnetic layer 13. The method forcalculating the average thickness of the magnetic layer 13 necessary forcalculating the volume of the magnetic layer 13 is as described above.

(Magnetic Viscosity Coefficient S)

First, a magnetic field of −1193 kA/m (15 kOe) is applied to the entiremagnetic recording medium 10 (measurement sample), and the magneticfield is returned to zero to obtain a residual magnetization state.Thereafter, a magnetic field equivalent to the value of the residualcoercive force Hr obtained from the DCD curve is applied in the oppositedirection. A magnetization amount is continuously measured at constanttime intervals for 1000 seconds in a state where a magnetic field isapplied. A magnetic viscosity coefficient S is calculated by comparing arelationship between time t and a magnetization amount M(t), obtained inthis way, with the following formula.M(t)=M0+S×In(t)

(In which M(t): magnetization amount at time t, M0: initialmagnetization amount, S: magnetic viscosity coefficient, ln(t): naturallogarithm of time)

(Friction Coefficient Ratio (μ_(B)/μ_(A)))

The magnetic recording medium 10 has a friction coefficient ratio(μ_(B)/μ_(A)) of preferably 1.0 or more and 2.0 or less, more preferably1.0 or more and 1.8 or less, still more preferably 1.0 or more and 1.6or less, in which μ_(A) represents a coefficient of dynamic frictionbetween the surface 13S of the magnetic layer 13 of the magneticrecording medium 10 and a magnetic head in a state where a tension of0.4 N is applied to the magnetic recording medium 10 in a longitudinaldirection thereof, and μ_(B) represents a coefficient of dynamicfriction between the surface 13S of the magnetic layer 13 of themagnetic recording medium 100 and the magnetic head in a state where atension of 1.2 N is applied to the magnetic recording medium 10 in thelongitudinal direction. The friction coefficient ratio (μ_(B)/μ_(A))within the above numerical range can reduce a change in the coefficientof dynamic friction due to the tension fluctuation during traveling, andtherefore can stabilize traveling of the magnetic recording medium 10.

The coefficient of dynamic friction μ_(A) and the coefficient of dynamicfriction μ_(B) for calculating the friction coefficient ratio(μ_(B)/μ_(A)) are determined as follows. First, as illustrated in FIG. 9, the magnetic recording medium 10 having a width of ½ inches is placedon two cylindrical guide rolls 91 and 92 each having a diameter of oneinch and disposed in parallel to and spaced apart from each other suchthat the surface 13S of the magnetic layer 13 is in contact with theguide rolls 91 and 92. The two guide rolls 91 and 92 have a fixedpositional relationship with each other.

Subsequently, the magnetic recording medium 10 is brought into contactwith a head block (for recording/reproducing) 93 mounted on an LTO5drive such that the surface 13S of the magnetic layer 13 is in contactwith the head block 93 and a holding angle θ1 [°] is 5.6°. One end ofthe magnetic recording medium 10 is held by a gripping jig 94 andconnected to a movable strain gauge 95, and a weight 96 is suspendedfrom the other end of the magnetic recording medium 10 to apply atension T0 of 0.4 N. Note that the head block 93 is fixed at a positionwhere the holding angle θ1 [°] is 5.6°. As a result, the positionalrelationship between the guide rolls 91 and 92 and the head block 93 isalso fixed.

Subsequently, the magnetic recording medium 10 is slid by 60 mm towardthe movable strain gauge 95 at a speed of 10 mm/s with respect to thehead block 93 by the movable strain gauge 95. An output value (voltage)of the movable strain gauge 95 at the time of sliding is converted intoT [N] on the basis of a linear relationship between an output valueacquired in advance and a load. T [N] is acquired 13 times from thestart of sliding to the end of sliding for the 60 mm slide describedabove, and 11 values of T [N] excluding totally two times of the firstand last times are simply averaged to obtain T_(ave)[N].

Thereafter, the coefficient of dynamic friction μ_(A) is determined bythe following formula.

$\begin{matrix}{\mu_{A} = {\frac{1}{\left( {\theta_{1}\lbrack{^\circ}\rbrack} \right) \times \left( {\prod{/180}} \right)} \times {\ln\left( \frac{T_{ave}\lbrack N\rbrack}{T_{0}\lbrack N\rbrack} \right)}}} & \left. \left\lbrack {{Numerical}{Formula}2} \right. \right\rbrack\end{matrix}$

The linear relationship described above is obtained as follows. That is,an output value (voltage) of the movable strain gauge 95 is obtained foreach of cases where a load of 0.4 N is applied to the movable straingauge 95 and a load of 1.5 N is applied thereto. From the obtained twooutput values and the two loads, a linear relationship between theoutput value and the load is obtained. Using the linear relationship, asdescribed above, the output value (voltage) from the movable straingauge 95 during sliding is converted into T [N].

The coefficient of dynamic friction μ_(B) is measured by the same methodas the method for measuring the coefficient of dynamic friction μ_(A)except that the tension To [N] applied to the other end is set to 1.2 N.

The friction coefficient ratio (μ_(B)/μ_(A)) is calculated from thecoefficient of dynamic friction μ_(A) and the coefficient of dynamicfriction μ_(B) measured as described above.

In a case where the coefficient of dynamic friction between the surface13S of the magnetic layer 13 and the magnetic head is represented byμ_(C) when a tension of 0.6 N is applied to the magnetic recordingmedium 10, a friction coefficient ratio (μC(1000)/μC(5)) between thefifth coefficient of dynamic friction μ_(C) (5) from the start of traveland the 1000th coefficient of dynamic friction μLC (1000) from the startof travel is preferably 1.0 or more and 1.9 or less, and more preferably1.2 or more and 1.8 or less. When the friction coefficient ratio(μC(1000)/μC(5)) is 1.0 or more and 1.9 or less, a change in thecoefficient of dynamic friction due to traveling many times can bereduced, and therefore traveling of the magnetic recording medium 10 canbe stabilized. Here, a magnetic head with a drive corresponding to themagnetic recording medium 10 is used as the magnetic head.

(Friction Coefficient Ratio (μ_(C(1000))/μ_(C(5))))

The coefficient of dynamic friction μC(5) and the coefficient of dynamicfriction μ_(C)(1000) for calculating the friction coefficient ratio(μC(1000)/μC(5)) are determined as follows.

The magnetic recording medium 10 has the friction coefficient ratio(μ_(C(1000))/μ_(C(5)))) of preferably 1.0 to 2.0, more preferably 1.0 to1.8, still more preferably 1.0 to 1.6, in which the friction coefficientratio (μ_(C(1000))/μ_(C(5)))) represents a friction coefficient ratiobetween the coefficient of dynamic friction μ_(C(5)) at the fifthreciprocation in a case where the magnetic recording medium in a statewhere a tension of 0.6 N is applied to the magnetic recording medium 10in a longitudinal direction thereof is reciprocatedly slid five times ona magnetic head and the coefficient of dynamic friction μ_(C(1000)) atthe 1000th reciprocation in a case where the magnetic recording medium10 is reciprocated 1000 times on the magnetic head. The frictioncoefficient ratio (μ_(C(1000))/μ_(C(5)))) within the above numericalrange can reduce a change in the coefficient of dynamic friction due totraveling many times, and therefore can stabilize traveling of themagnetic recording medium 10.

The coefficient of dynamic friction μ_(C(5)) and the coefficient ofdynamic friction μ_(C(1000)) for calculating the friction coefficientratio (μ_(C(1000))/μ_(C(5))) are determined as follows.

The magnetic recording medium 10 is connected to a movable strain gauge71 in the same manner as the method for measuring the coefficient ofdynamic friction μ_(A) except that the tension T₀ [N] applied to theother end of the magnetic recording medium 10 is set to 0.6 N. Then, themagnetic recording medium 10 is slid 60 mm toward the movable straingauge at 10 mm/s with respect to the head block 74 (forward path) andslid 60 mm away from the movable strain gauge (return path). Thisreciprocating operation is repeated 1000 times. Among the 1000reciprocating operations, a strain gauge output value (voltage) isacquired 13 times from the start of sliding to the end of sliding forthe 60 mm slide in the fifth forward path, and the output value isconverted into T [N] on the basis of a linear relationship (describedlater) between an output value determined for the coefficient of dynamicfriction μ_(A) and a load. Eleven values of T [N] excluding totally twotimes of the first and last times are simply averaged to determineT_(ave)[N].

The coefficient of dynamic friction μ_(C(5)) is determined by thefollowing formula.

$\begin{matrix}{\mu_{C(5)} = {\frac{1}{\left( {\theta_{1}\lbrack{^\circ}\rbrack} \right) \times \left( {\pi/180} \right)} \times {\ln\left( \frac{T_{ave}\lbrack N\rbrack}{T_{0}\lbrack N\rbrack} \right)}}} & \left. \left\lbrack {{Numerical}{Formula}3} \right. \right\rbrack\end{matrix}$

Moreover, the coefficient of dynamic friction μ_(C(1000)) is determinedin a similar manner to the coefficient of dynamic friction μ_(C(5))except that measurement is performed for the 1000th forward path.

The friction coefficient ratio μ_(C(1000))/μ_(C(5)) is calculated fromthe coefficient of dynamic friction μ_(C(5)) and the coefficient ofdynamic friction μ_(C(1000)) measured as described above.

[1-2 Method for Manufacturing Magnetic Recording Medium 10]

Next, a method for manufacturing the magnetic recording medium 10 havingthe above-described configuration will be described. First, by kneadingand dispersing nonmagnetic powder, a binder, a lubricant, and the likein a solvent, a base layer forming coating material is prepared. Next,by kneading and dispersing magnetic powder, a binder, a lubricant, andthe like in a solvent, a magnetic layer forming coating material isprepared. Next, by kneading and dispersing a binder, nonmagnetic powder,and the like in a solvent, a back layer forming coating material isprepared. For preparing the magnetic layer forming coating material, thebase layer forming coating material, and the back layer forming coatingmaterial, for example, the following solvents, dispersing devices, andkneading devices can be used.

Examples of the solvent used for preparing the above-described coatingmaterial include a ketone-based solvent such as acetone, methyl ethylketone, methyl isobutyl ketone, or cyclohexanone, an alcohol-basedsolvent such as methanol, ethanol, or propanol, an ester-based solventsuch as methyl acetate, ethyl acetate, butyl acetate, propyl acetate,ethyl lactate, or ethylene glycol acetate, an ether-based solvent suchas diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran,or dioxane, an aromatic hydrocarbon-based solvent such as benzene,toluene, or xylene, a halogenated hydrocarbon-based solvent such asmethylene chloride, ethylene chloride, carbon tetrachloride, chloroform,or chlorobenzene, and the like. These solvents may be used singly, ormay be used in a mixture thereof appropriately.

Examples of a kneading device used for preparing the above-describedcoating material include a continuous twin-screw kneading machine, acontinuous twin-screw kneading machine capable of performing dilution inmultiple stages, a kneader, a pressure kneader, a roll kneader, and thelike, but are not particularly limited to these devices. Furthermore,examples of a dispersing device used for preparing the above-describedcoating material include a roll mill, a ball mill, a horizontal sandmill, a vertical sand mill, a spike mill, a pin mill, a tower mill, apearl mill (for example, “DCP mill” manufactured by Eirich Co., Ltd. andthe like), a homogenizer, an ultrasonic wave dispersing machine, and thelike, but are not particularly limited to these devices.

Next, the base layer forming coating material is applied to one mainsurface 11A of the substrate 11 and dried to form the base layer 12.Subsequently, by applying the magnetic layer forming coating materialonto the base layer 12 and drying the magnetic layer forming coatingmaterial, the magnetic layer 13 is formed on the base layer 12. Notethat during drying, magnetic powder is preferably subjected to magneticfield orientation in the thickness direction of the substrate 11 by, forexample, a solenoid coil. Furthermore, during drying, the magneticpowder may be subjected to magnetic field orientation in a travelingdirection (longitudinal direction) of the substrate 11 by, for example,a solenoid coil, and then may be subjected to magnetic field orientationin a thickness direction of the substrate 11. By performing such amagnetic field orientation treatment, the degree of vertical orientation(that is, squareness ratio S1) of the magnetic powder can be improved.After the magnetic layer 13 is formed, the back layer forming coatingmaterial is applied to the other main surface 11B of the substrate 11and dried to form the back layer 14. As a result, the magnetic recordingmedium 10 is obtained.

The squareness ratios S1 and S2 are set to desired values, for example,by adjusting the intensity of a magnetic field applied to a coating filmof the magnetic layer forming coating material, the concentration of asolid content in the magnetic layer forming coating material, and dryingconditions (drying temperature and drying time) of the coating film ofthe magnetic layer forming coating material. The intensity of a magneticfield applied to a coating film is preferably at least twice thecoercive force of the magnetic powder. In order to further increase thesquareness ratio S1 (that is, to further reduce the squareness ratioS2), it is preferable to improve the dispersion state of the magneticpowder in the magnetic layer forming coating material. Furthermore, inorder to further increase the squareness ratio S1, it is also effectiveto magnetize the magnetic powder before the magnetic layer formingcoating material is put into an orientation device for magnetic fieldorientation of the magnetic powder. Note that the above methods foradjusting the squareness ratios S1 and S2 may be used singly or incombination of two or more thereof.

Thereafter, the obtained magnetic recording medium 10 is calendered tosmooth the surface 13S of the magnetic layer 13. Next, the magneticrecording medium 10 that has been calendered is wound into a roll shape.Thereafter, the magnetic recording medium 10 is heated in this state,and the large number of protrusions 14A on the surface 14S of the backlayer 14 are thereby transferred onto the surface 13S of the magneticlayer 13. As a result, the large number of recesses 13A are formed onthe surface 13S of the magnetic layer 13.

The temperature of the heat treatment is preferably 50° C. or higher and80° C. or lower. When the temperature of the heat treatment is 50° C. orhigher, good transferability can be obtained. Meanwhile, when thetemperature of the heat treatment is 80° C. or lower, the amount ofpores may be excessively increased, and the lubricant on the surface 13Sof the magnetic layer 13 may be excessive. Here, the temperature of theheat treatment is the temperature of an atmosphere holding the magneticrecording medium 10.

Time for the heat treatment is preferably 15 hours or more and 40 hoursor less. When the time for heat treatment is 15 hours or more, goodtransferability can be obtained. Meanwhile, when the time for heattreatment is 40 hours or less, a decrease in productivity can besuppressed.

Furthermore, a range of pressure applied to the magnetic recordingmedium 10 during the heat treatment is preferably 150 kg/cm or more and400 kg/cm or less.

Finally, the magnetic recording medium 10 is cut into a predeterminedwidth (for example, a width of ½ inches). As a result, the targetmagnetic recording medium 10 is obtained.

In the above manufacturing method, the large number of protrusions 14Aformed on the surface 14S of the back layer 14 are transferred onto thesurface 13S of the magnetic layer 13, and pores (recesses 13A) arethereby formed on the surface of the magnetic layer 13. However, themethod for forming the pores is not limited thereto. For example, poresmay be formed on the surface 13S of the magnetic layer 13 by adjustingthe type of a solvent included in the magnetic layer forming coatingmaterial and/or adjusting drying conditions of the magnetic layerforming coating material. Furthermore, for example, in the process ofdrying the solvent of the magnetic layer forming coating material, porescan be formed by an uneven distribution of the solid and the solventincluded in the magnetic layer forming coating material. Furthermore, inthe process of applying the magnetic layer forming coating material, thesolvent included in the magnetic layer forming coating material can alsobe absorbed by the base layer 12 through the pores of the base layer 12formed when the lower layer is applied and dried. In the drying stepafter the application, the solvent moves from the base layer 12 throughthe magnetic layer 13, and pores connecting the magnetic layer 13 to thebase layer 12 can be thereby formed.

[1-3. Configuration of Recording/Reproducing Device 30]

Next, the configuration of the recording/reproducing device 30 forrecording information on the magnetic recording medium 10 describedabove and reproducing information from the magnetic recording medium 10described above will be described with reference to FIG. 6 .

The recording/reproducing device 30 can adjust a tension applied to themagnetic recording medium 10 in a longitudinal direction thereof.Furthermore, the recording/reproducing device 30 can load the magneticrecording medium cartridge 10A thereon. Here, for ease of explanation, acase where the recording/reproducing device 30 can load one magneticrecording medium cartridge 10A thereon will be described. However, inthe present disclosure, the recording/reproducing device 30 can load aplurality of magnetic recording medium cartridges 10A thereon. Asdescribed above, the magnetic recording medium 10 has a tape shape, andmay be, for example, a long magnetic recording tape. The magneticrecording medium 10 may be housed in a casing in a state of being woundaround a reel inside the magnetic recording medium cartridge 10A, forexample. The magnetic recording medium 10 travels in the longitudinaldirection during recording and reproduction. Furthermore, the magneticrecording medium 10 can record a signal at the shortest recordingwavelength of preferably 100 nm or less, more preferably 75 nm or less,still more preferably 60 nm or less, particularly preferably 50 nm orless, and can be used, for example, for the recording/reproducing device30 having the shortest recording wavelength within the above range. Therecording track width can be, for example, 2 μm or less.

The recording/reproducing device 30 is connected to informationprocessing devices such as a server 41 and a personal computer(hereinafter referred to as “PC”) 42, for example, through a network 43,and data supplied from these information processing devices can berecorded in the magnetic recording medium cartridge 10A.

As illustrated in FIG. 6 , the recording/reproducing device 30 includesa spindle 31, a reel 32, a driving device 33, a driving device 34, aplurality of guide rollers 35, a head unit 36, a communication interface(hereinafter referred to as I/F) 37, and a control device 38.

The spindle 31 can mount the magnetic recording medium cartridge 10Athereon. The magnetic recording medium cartridge 10A complies with thelinear tape open (LTO) standard, and rotatably houses a single reel 10Cin which the magnetic recording medium 10 is wound in a cartridge case10B. A V-shaped servo pattern is recorded in advance as a servo signalon the magnetic recording medium 10. The reel 32 can fix a tip of themagnetic recording medium 10 pulled out from the magnetic recordingmedium cartridge 10A.

The driving device 33 rotationally drives the spindle 31. The drivingdevice 34 rotationally drives the reel 32. When data is recorded orreproduced on the magnetic recording medium 10, the driving device 33and the driving device 34 rotationally drive the spindle 31 and the reel32, respectively, to cause the magnetic recording medium 10 to travel.The guide roller 35 is a roller for guiding traveling of the magneticrecording medium 10.

The head unit 36 includes a plurality of recording heads for recordingdata signals on the magnetic recording medium 10, a plurality ofreproducing heads for reproducing data signals recorded on the magneticrecording medium 10, and a plurality of servo heads for reproducingservo signals recorded on the magnetic recording medium 10. As therecording head, for example, a ring type head can be used, and as thereproducing head, for example, a magnetoresistive effect type magnetichead can be used. However, the types of the recording head andreproducing head are not limited thereto.

The I/F 37 is for communicating with an information processing devicesuch as the server 41 or the PC 42, and is connected to the network 43.

The control device 38 controls the entire recording/reproducing device30. For example, the control device 38 causes the head unit 36 to recorda data signal supplied from an information processing device such as theserver 41 or the PC 42 on the magnetic recording medium 10 in responseto a request from the information processing device. Furthermore, thecontrol device 38 causes the head unit 36 to reproduce the data signalrecorded on the magnetic recording medium 10 in response to a requestfrom an information processing device such as the server 41 or the PC 42and supplies the data signal to the information processing device.

[1-4 Effect]

As described above, the magnetic recording medium 10 of the presentembodiment is a tape-shaped member in which the substrate 11, the baselayer 12, the magnetic layer 13, and the back layer 14 are sequentiallylaminated, and satisfies the following constituent requirements (1) to(9).

(1) The substrate 11 includes a polyester as a main component.

(2) The kurtosis (Sku) of the surface 14S of the back layer 14 oppositeto the substrate is 2.0 or more.

(3) On the surface 13S of the magnetic layer 13, the recesses 13A havinga depth of 20% or more of the average thickness of the magnetic layer 13are formed at a ratio of 10 or more and 200 or less per 1600 μm².

(4) The arithmetic average roughness Ra of the surface 13S of themagnetic layer 13 is 2.5 nm or less.

(5) The base layer 12 and the magnetic layer 13 each include alubricant.

(6) The BET specific surface area of the entire magnetic recordingmedium 10 is 3.5 m²/g or more in a state where the lubricant has beenremoved from the magnetic recording medium 10 and the magnetic recordingmedium 10 has been dried.

(7) The squareness ratio in the perpendicular direction is 65% or more.

(8) The average thickness of the magnetic layer 13 is 90 nm or less.

(9) The average thickness of the magnetic recording medium 10 is 5.6 μmor less.

Because of having such a configuration, the magnetic recording medium 10of the present embodiment can suppress an increase in the coefficient ofdynamic friction even after the total thickness is reduced and repeatedrecording or repeated reproduction is executed. Furthermore, since thekurtosis of the surface 14S of the back layer 14 in contact with thesurface 13S of the magnetic layer 13 is 2.0 or more, an appropriatefrictional force is generated also at an interface between the surface13S and the surface 14S where the lubricant is likely to appear, andwinding deviation of the magnetic recording medium 10 can be prevented.Furthermore, since a plurality of recesses 13A is formed on the surface13S of the magnetic layer 13 at a predetermined surface density,specifically, since the recesses 13A having a depth of 20% or more ofthe average thickness of the magnetic layer 13 are formed at a ratio of10 or more and 200 or less per 1600 μm², good electromagnetic conversioncharacteristics can be maintained. It is considered that this is becauseby forming the plurality of recesses 13A with an appropriate surfacedensity, air trapped while the magnetic recording medium 10 is travelingcan be released from the interface between the surface 13S of themagnetic layer 13 and the head, and contact between the surface 13S ofthe magnetic layer 13 and the head and can be kept well.

<2. Modification>

(Modification 1)

In the embodiment described above, the ε iron oxide particle 20 (FIG. 4) having the two-layered shell portion 22 has been illustrated anddescribed, but the magnetic recording medium of the present technologymay include, for example, as illustrated in FIG. 7 , an ε iron oxideparticle 20A having a single-layered shell portion 23. The shell portion23 in the ε iron oxide particle 20A has a similar configuration to thefirst shell portion 22 a, for example. However, the ε iron oxideparticle 20 having the two-layered shell portion 22 described in theembodiment described above is more preferable than the ε iron oxideparticle 20A of Modification 1 from a viewpoint of suppressingcharacteristic deterioration.

(Modification 2)

In the magnetic recording medium 10 according to the embodimentdescribed above, the case where the ε iron oxide particle 20 having acore-shell structure has been illustrated and described. However, the εiron oxide particle may include an additive instead of the core-shellstructure, or may have a core-shell structure and include an additive.In this case, some of Fe atoms in the ε iron oxide particles arereplaced with an additive. Even by inclusion of an additive in an ε ironoxide particle, a coercive force Hc of the entire ε iron oxide particlescan be adjusted to a coercive force Hc suitable for recording.Therefore, recordability can be improved. The additive is a metalelement other than iron, preferably a trivalent metal element, morepreferably at least one of aluminum (Al), gallium (Ga), and indium (In),and still more preferably at least one of Al and Ga.

Specifically, the ε iron oxide including an additive is an ε—Fe₂—xMxO₃crystal (in which M represents a metal element other than iron,preferably a trivalent metal element, more preferably at least one ofAl, Ga, and In, and still more preferably at least one of Al and Ga, andx satisfies, for example, 0<x<1).

(Modification 3)

The magnetic powder of the present disclosure may include powder ofnanoparticles including hexagonal ferrite (hereinafter referred to as“hexagonal ferrite particles”) instead of the powder of ε iron oxideparticles. The hexagonal ferrite particle has, for example, a hexagonalplate shape or a substantially hexagonal plate shape. The hexagonalferrite preferably includes at least one of barium (Ba), strontium (Sr),lead (Pb), and calcium (Ca), more preferably at least one of Ba and Sr.Specifically, the hexagonal ferrite may be, for example, barium ferriteor strontium ferrite. The barium ferrite may further include at leastone of Sr, Pb, and Ca in addition to Ba. The strontium ferrite mayfurther include at least one of Ba, Pb, and Ca in addition to Sr.

More specifically, the hexagonal ferrite has an average compositionrepresented by a general formula MFe₁₂O₁₉. However, M represents atleast one metal of Ba, Sr, Pb, and Ca, preferably at least one metal ofBa and Sr, for example. M may represent a combination of Ba and one ormore metals selected from the group consisting of Sr, Pb, and Ca.Furthermore, M may represent a combination of Sr and one or more metalsselected from the group consisting of Ba, Pb, and Ca. In the abovegeneral formula, some of Fe atoms may be replaced with other metalelements.

In a case where the magnetic powder includes powder of hexagonal ferriteparticles, the average particle size of the magnetic powder ispreferably 50 nm or less, more preferably 40 nm or less, and still morepreferably 30 nm or less. The average particle size of the magneticpowder is more preferably 25 nm or less, 22 nm or less, 21 nm or less,or 20 nm or less. Furthermore, the average particle size of the magneticpowder is, for example, 10 nm or more, preferably 12 nm or more, andmore preferably 15 nm or more. Therefore, the average particle size ofthe magnetic powder including powder of hexagonal ferrite particles canbe, for example, 10 nm or more and 50 nm or less, 10 nm or more and 40nm or less, 12 nm or more and 30 nm or less, 12 nm or more and 25 nm orless, or 15 nm or more and 22 nm or less. In a case where the averageparticle size of the magnetic powder is the above upper limit value orless (for example, 50 nm or less, particularly 30 nm or less), in themagnetic recording medium 10 having a high recording density, goodelectromagnetic conversion characteristics (for example, SNR) can beobtained. In a case where the average particle size of the magneticpowder is the above lower limit value or more (for example, 10 nm ormore, preferably 12 nm or more), the dispersibility of the magneticpowder is further improved, and better electromagnetic conversioncharacteristics (for example, SNR) can be obtained.

In a case where the magnetic powder includes hexagonal ferriteparticles, the average aspect ratio of the magnetic powder can bepreferably 1 or more and 3.5 or less, more preferably 1 or more and 3.1or less, or 2 or more and 3.1 or less, and still more preferably 2 ormore and 3 or less. When the average aspect ratio of the magnetic powderis within the above numerical range, aggregation of the magnetic powdercan be suppressed, and moreover, resistance applied to the magneticpowder can be suppressed when the magnetic powder is vertically orientedin a step of forming the magnetic layer 13. This can improve thevertical orientation of the magnetic powder.

Note that the average particle size and average aspect ratio of themagnetic powder including powder of hexagonal ferrite particles aredetermined as follows. First, the magnetic recording medium 10 to bemeasured is processed to be thinned by a focused ion beam (FIB) methodand the like. Thinning is performed in the length direction(longitudinal direction) of the magnetic tape. Cross-sectionalobservation is performed for the obtained thin sample such that theentire recording layer is included with respect to the thicknessdirection of the recording layer using a transmission electronmicroscope (H-9500 manufactured by Hitachi High-Technologies) with anacceleration voltage of 200 kV and an overall magnification of 500,000times. Next, from the imaged TEM photograph, 50 particles each having aside surface directed to an observation surface are selected, andmaximum plate thicknesses DA of the particles are measured. The maximumplate thicknesses DA thus determined are simply averaged (arithmeticallyaveraged) to determine an average maximum plate thickness DAave.Subsequently, plate diameters DB of the particles of the magnetic powderare measured. Here, the plate diameter DB means the largest distanceamong distances between two parallel lines drawn from all angles so asto come into contact with an outline of each of the particles of themagnetic powder (so-called maximum Feret diameter). Subsequently, themeasured plate diameters DB are simply averaged (arithmeticallyaveraged) to determine an average plate diameter DBave. Then, an averageaspect ratio (DBave/DAave) of the particles is determined from theaverage maximum plate thickness DAave and the average plate diameterDBave.

In a case where the magnetic powder includes powder of hexagonal ferriteparticles, the average particle volume of the magnetic powder ispreferably 5900 nm³ or less, more preferably 500 nm³ or more and 3400nm³ or less, and still more preferably 1000 nm³ or more and 2500 nm³ orless. When the average particle volume of the magnetic powder is 5900nm³ or less, a similar effect to that in a case where the averageparticle size of the magnetic powder is 30 nm or less can be obtained.Meanwhile, when the average particle volume of the magnetic powder is500 nm³ or more, a similar effect to a case where the average particlesize of the magnetic powder is 12 nm or more can be obtained.

Note that the average particle volume of the magnetic powder isdetermined as follows. First, the average maximum plate thickness DAaveand the average maximum plate diameter DBave are determined by theabove-described method for calculating the average particle size of themagnetic powder. Next, an average volume V of the ε iron oxide particlesis determined by the following formula.

$\begin{matrix}{V = {\frac{3\sqrt{3}}{8} \times {DA}_{ave} \times {DB}_{ave} \times {DB}_{ave}}} & \left. \left\lbrack {{Numerical}{Formula}4} \right. \right\rbrack\end{matrix}$

According to a particularly preferable embodiment of the presenttechnology, the magnetic powder can be barium ferrite magnetic powder orstrontium ferrite magnetic powder, and more preferably barium ferritemagnetic powder. Barium ferrite magnetic powder includes iron oxidemagnetic particles having barium ferrite as a main phase (hereinafterreferred to as “barium ferrite particles”). Barium ferrite magneticpowder has high data recording reliability. For example, barium ferritemagnetic powder keeps coercive force even in a high-temperature andhigh-humidity environment. Barium ferrite magnetic powder is preferableas magnetic powder from such a viewpoint.

The average particle size of barium ferrite magnetic powder is 50 nm orless, more preferably 10 nm or more and 40 nm or less, and still morepreferably 12 nm or more and 25 nm or less.

In a case where the magnetic layer 13 includes barium ferrite magneticpowder as magnetic powder, the average thickness tm [nm] of the magneticlayer 13 preferably satisfies 35 nm≤tm≤100 nm, and is particularlypreferably 80 nm or less. Furthermore, the magnetic recording medium 10has a coercive force Hc of preferably 160 kA/m or more and 280 kA/m orless, more preferably 165 kA/m or more and 275 kA/m or less, still morepreferably 170 kA/m or more and 270 kA/m or less when the coercive forceHc is measured in a thickness direction (perpendicular direction) of themagnetic recording medium 10.

(Modification 4)

The magnetic powder may include powder of nanoparticles includingCo-containing spinel ferrite (hereinafter referred to as “cobalt ferriteparticles”) instead of the powder of ε iron oxide particles. The cobaltferrite particle preferably has uniaxial crystal anisotropy. The cobaltferrite particle has, for example, a cubic shape or a substantiallycubic shape. The Co-containing spinel ferrite may further include atleast one of Ni, Mn, Al, Cu, and Zn in addition to Co.

The Co-containing spinel ferrite has, for example, an averagecomposition represented by the following formula.Co_(x) M _(y)Fe₂O_(z)

(Provided that in formula (1), M represents, for example, at least onemetal of Ni, Mn, Al, Cu, and Zn). x represents a value within a range of0.4≤x≤1.0. y represents a value within a range of 0≤y≤0.3. Provided thatx and y satisfy a relationship of (x+y)≤1.0. z represents a value withina range of 3≤z≤4. Some of Fe atoms may be replaced with another metalelement.)

In a case where the magnetic powder includes powder of cobalt ferriteparticles, the average particle size of the magnetic powder ispreferably 25 nm or less, and more preferably 10 nm or more and 23 nm orless. When the average particle size of the magnetic powder is 25 nm orless, good electromagnetic conversion characteristics (for example, SNR)can be obtained in the magnetic recording medium 10 having a highrecording density. Meanwhile, when the average particle size of themagnetic powder is 10 nm or more, dispersibility of the magnetic powderis further improved, and better electromagnetic conversioncharacteristics (for example, SNR) can be obtained. In a case where themagnetic powder includes powder of cobalt ferrite particles, the averageaspect ratio of the magnetic powder is similar to that of the embodimentdescribed above. Furthermore, a method for calculating the averageparticle size and the average aspect ratio of the magnetic powder isdetermined in a similar manner to that of the embodiment describedabove.

The average particle volume of the magnetic powder is preferably 15000nm³ or less, and more preferably 1000 nm³ or more and 12000 nm³ or less.When the average particle volume of the magnetic powder is 15000 nm³ orless, a similar effect to that in a case where the average particle sizeof the magnetic powder is 25 nm or less can be obtained. Meanwhile, whenthe average particle volume of the magnetic powder is 1000 nm³ or more,a similar effect to a case where the average particle size of themagnetic powder is 10 nm or more can be obtained. Note that a method forcalculating the average particle volume of the magnetic powder issimilar to the method for calculating the average particle volume of themagnetic powder (the method for calculating the average particle volumein a case where the ε iron oxide particle has a cubic shape or asubstantially cubic shape) in the embodiment described above.

The coercive force Hc of cobalt ferrite magnetic powder is preferably2500 Oe or more, and more preferably 2600 Oe or more and 3500 Oe orless.

(Modification 5)

The magnetic recording medium 10 may further include a barrier layer 15disposed on at least one surface of the substrate 11, for example, asillustrated in FIG. 8 . The barrier layer 15 is a layer for suppressinga dimensional change according to an environment of the substrate 11.Examples of a cause of the dimensional change include a hygroscopicproperty of the substrate 11. However, by disposing the barrier layer15, a penetration rate of moisture into the substrate 11 can be reduced.The barrier layer 15 includes, for example, a metal or a metal oxide. Asthe metal herein, for example, at least one of Al, Cu, Co, Mg, Si, Ti,V, Cr, Mn, Fe, Ni, Zn, Ga, Ge, Y, Zr, Mo, Ru, Pd, Ag, Ba, Pt, Au, and Tacan be used. As the metal oxide, for example, a metal oxide includingone or more of the above metals can be used. More specifically, forexample, at least one of Al₂O₃, CuO, CoO, SiO₂, Cr₂O₃, TiO₂, Ta₂O₅, andZrO₂ can be used. Furthermore, the barrier layer 15 may includediamond-like carbon (DLC), diamond, and the like.

The average thickness of the barrier layer 15 is preferably 20 nm ormore and 1000 nm or less, and more preferably 50 nm or more and 1000 nmor less. The average thickness of the barrier layer 15 is determined ina similar manner to the average thickness of the magnetic layer 13.However, a magnification of a TEM image is appropriately adjustedaccording to the thickness of the barrier layer 15.

(Modification 6)

In the embodiment described above, the case where the large number ofrecesses 13A are formed on the surface 13S of the magnetic layer 13 bytransferring the large number of protrusions 14A formed on the surface14S of the back layer 14 onto the surface 13S of the magnetic layer 13has been described. However, the method for forming the large number ofrecesses 13A is not limited thereto. For example, the large number ofrecesses 13A may be formed on the surface 13S of the magnetic layer 13by adjusting the type of a solvent included in the magnetic layerforming coating material, drying conditions of the magnetic layerforming coating material, and the like.

(Modification 7)

The magnetic recording medium 10 according to the embodiment describedabove may be used for a library device. In this case, the library devicemay include a plurality of the recording/reproducing devices 30 in theembodiment described above.

(Modification 8)

(Elasticity and Tension Control of Magnetic Recording Medium 10)

A modification in which the tension of the magnetic recording medium 10is controlled by the elasticity of the magnetic recording medium 10 andthe recording/reproducing device 30 will be described. In the LTOstandard, the number of recording tracks is increasing rapidly due to ademand for high-density recording of data. In such a case, a recordingtrack width is narrow, and a slight fluctuation in the width (Y-axisdirection) of the magnetic recording medium 1 may be a problem.

For example, predetermined data is stored in the magnetic recordingmedium 1 by a data recording device 20, and then (for example, afterstorage for a certain period of time) the data recorded on the magneticrecording medium 10 is reproduced by the recording/reproducing device30. In such a case, when the width of the magnetic recording medium 10at the time of data reproduction fluctuates slightly compared to thewidth of the magnetic recording medium 10 at the time of data recording,off-track may occur (a data reproducing head in the head unit 36 may bepositioned on a wrong recording track 5). For this reason, there is apossibility that data recorded on the magnetic recording medium 10cannot be accurately reproduced and an error occurs.

Examples of a cause of the fluctuation in the width of the magneticrecording medium 10 include fluctuation in temperature, fluctuation inhumidity, and the like. In general, a technique is used in which themagnetic recording medium 10 is designed so as not to expand andcontract to cope with fluctuation in the width of the magnetic recordingmedium 10. However, it is practically impossible to prevent the magneticrecording medium 10 from expanding and contracting completely.

Therefore, this modification uses a technique which does not make itdifficult for the magnetic recording medium 10 to expand and contract,but conversely, makes it easy for the magnetic recording medium 10 toexpand and contract to some extent, and controls (increases ordecreases) the tension of the magnetic recording medium 10 (X-axisdirection: tension of the magnetic recording medium 10 in a conveyancedirection thereof) in the recording/reproducing device 30.

Specifically, the data recording device 20 increases the tension of themagnetic recording medium 10 in the longitudinal direction (X-axisdirection) as necessary (in a case where the width of the magneticrecording medium 10 is widened) during reproduction of a data signal toreduce the width (Y-axis direction) of the magnetic recording medium 10.Furthermore, the recording/reproducing device 30 reduces the tension ofthe magnetic recording medium 10 in the longitudinal direction asnecessary (in a case where the width of the magnetic recording medium 10is narrowed) during reproduction of a data signal to increase the widthof the magnetic recording medium 10. Note that the recording/reproducingdevice 30 may control the tension of the magnetic recording medium 10 inthe longitudinal direction not only during reproduction of a data signalbut also during recording of the data signal.

According to such a method, for example, when the width of the magneticrecording medium 10 has fluctuated due to temperature and the like, byadjusting the width of the magnetic recording medium 10 as necessary,the width of the magnetic recording medium 10 can be constant.Therefore, it is considered that off-track can be prevented and datarecorded on the magnetic recording medium 10 can be accuratelyreproduced.

EXAMPLES

Hereinafter, the present disclosure will be described specifically withExamples, but the present disclosure is not limited only to theseExamples.

In the following Examples and Comparative Examples, the squareness ratioS1 in the perpendicular direction, the squareness ratio S2 in thelongitudinal direction, the pore distribution (pore volume and porediameter of the maximum pore volume at the time of desorption), the BETspecific surface area, the average aspect ratio, the average particlesize of the magnetic powder, the average particle volume of the magneticpowder, the average thickness of the magnetic layer, the averagethickness of the substrate, the arithmetic average roughness of thesurface of the magnetic layer, the kurtosis of the surface of the backlayer, and the surface density of the recesses of the surface of themagnetic layer are values determined by the measurement method describedin the embodiment described above.

Example 1

A magnetic recording medium as Example 1 was obtained as follows.

<Step of Preparing Magnetic Layer Forming Coating Material>

A magnetic layer forming coating material was prepared as follows.First, a first composition having the following formulation was kneadedwith an extruder. Next, the kneaded first composition and a secondcomposition having the following formulation were added to a stirringtank equipped with a disper, and were premixed. Subsequently, themixture was further subjected to sand mill mixing, and was subjected toa filter treatment to prepare a magnetic layer forming coating material.

(First Composition)

Each component and weight in the first composition are as follows.

-   -   Powder of barium ferrite (BaFe₁₂O₁₉) particles (hexagonal plate        shape, average aspect ratio 2.8, average particle volume 1950        nm³): 100 parts by mass    -   Vinyl chloride-based resin: 42 parts by mass (including a        solvent)

(Resin solution: resin content 30% by mass, cyclohexanone 70% by mass)

(Degree of polymerization: 300, Mn=10000, OSO3K=0.07 mmol/g andsecondary OH=0.3 mmol/g were included as polar groups)

-   -   Aluminum oxide powder: 5 parts by mass (α—Al₂O₃, average        particle diameter 0.1 μm)    -   Carbon black (manufactured by Tokai Carbon Co., Ltd., trade        name: Seast TA): 2 parts by mass

(Second Composition)

Each component and weight in the second composition are as follows.

-   -   Vinyl chloride-based resin: 3 parts by mass (including a        solvent)

(Resin solution: resin content 30% by mass, cyclohexanone 70% by mass)

-   -   n-Butyl stearate as a fatty acid ester: 2 parts by mass    -   Methyl ethyl ketone: 121.3 parts by mass    -   Toluene: 121.3 parts by mass    -   Cyclohexanone: 60.7 parts by mass

To the magnetic layer forming coating material prepared as describedabove, 4 parts by mass of polyisocyanate (trade name: Coronate L,manufactured by Tosoh Corporation) as a curing agent and 2 parts by massof stearic acid as a fatty acid were added.

<Step of Preparing Base Layer Forming Coating Material>

A base layer forming coating material was prepared as follows. First, athird composition having the following formulation was kneaded with anextruder. Next, the kneaded third composition and a fourth compositionhaving the following formulation were added to a stirring tank equippedwith a disper, and were premixed. Subsequently, the mixture was furthersubjected to sand mill mixing, and was subjected to a filter treatmentto prepare a base layer forming coating material.

(Third Composition)

Each component and weight in the third composition are as follows.

-   -   Acicular iron oxide powder (α—Fe₂O₃, average major axis length        0.15 μm): 100 parts by mass    -   Vinyl chloride-based resin (resin solution: resin content 30% by        mass, cyclohexanone 70% by mass): 60.6 parts by mass (including        a solution)    -   Carbon black (average particle diameter 20 nm): 10 parts by mass

(Fourth Composition)

Each component and weight in the fourth composition are as follows.

-   -   Polyurethane-based resin UR8200 (manufactured by Toyobo Co.,        Ltd.): 18.5 parts by mass    -   n-Butyl stearate as a fatty acid ester: 2 parts by mass    -   Methyl ethyl ketone: 108.2 parts by mass    -   Toluene: 108.2 parts by mass    -   Cyclohexanone: 18.5 parts by mass

To the base layer forming coating material prepared as described above,4 parts by mass of polyisocyanate (trade name: Coronate L, manufacturedby Tosoh Corporation) as a curing agent and 2 parts by mass of stearicacid as a fatty acid were added.

<Step of preparing back layer forming coating material>

A back layer forming coating material was prepared as follows. Thefollowing raw materials were mixed in a stirring tank equipped with adisper, and were subjected to filter treatment to prepare a back layerforming coating material.

-   -   Carbon black powder having a small particle diameter (average        particle diameter (D50) 20 nm): 90 parts by mass    -   Carbon black powder having a large particle diameter (average        particle diameter (D50) 270 nm): 10 parts by mass    -   Polyester polyurethane (manufactured by Tosoh Corporation, trade        name: N-2304): 100 parts by mass    -   Methyl ethyl ketone: 500 parts by mass    -   Toluene: 400 parts by mass    -   Cyclohexanone: 100 parts by mass

<Application Step>

Using the magnetic layer forming coating material and the base layerforming coating material prepared as described above, a base layer and amagnetic layer were formed on one main surface of a long polyester filmhaving an average thickness of 4.0 μm as a nonmagnetic support such thatthe average thickness of the base layer was 0.6 μm and the averagethickness of the magnetic layer was 80 nm after calendering as follows.First, the base layer forming coating material was applied onto one mainsurface of the polyester film and dried to form a base layer. Next, themagnetic layer forming coating material was applied onto the base layerand dried to form a magnetic layer. Note that the magnetic powder wassubjected to magnetic field orientation in a thickness direction of thefilm by a solenoid coil when the magnetic layer forming coating materialwas dried. Furthermore, drying conditions (drying temperature and dryingtime) of the magnetic layer forming coating material were adjusted, andthe squareness ratio S1 of the magnetic recording medium in thethickness direction (perpendicular direction) and the squareness ratioS2 thereof in the longitudinal direction were set to the valuesillustrated in Table 2. Subsequently, the back layer forming coatingmaterial was applied onto the other main surface of the polyester filmand dried to form a back layer having an average thickness of 0.3 μm.Furthermore, the kurtosis of the surface of the back layer was set to2.3. As a result, a magnetic recording medium was obtained. Note thatthe kurtosis of the surface of the back layer can be adjusted bychanging the size and shape of a particle added to the back layerforming coating material. Moreover, for example, the kurtosis of thesurface of the back layer can also be adjusted by adjusting the pressureand temperature during calendering described later after the back layeris formed. For example, by increasing the pressure in calendering, thekurtosis of the surface of the back layer can be reduced. Furthermore,also by raising the temperature in calendering, the kurtosis of thesurface of the back layer can be reduced.

<Calendering Step and Transfer Step>

Subsequently, calendering was performed to smooth a surface of themagnetic layer. Next, the magnetic recording medium having a smoothsurface of the magnetic layer was wound into a roll shape, and then themagnetic recording medium was heated at 60° C. for 10 hours in thisstate. Then, the magnetic recording medium was rewound in a roll shapesuch that an end located on an inner circumferential side was located onan outer circumferential side oppositely, and then the magneticrecording medium was heated again at 60° C. for 10 hours in this state.As a result, a large number of protrusions on the surface of the backlayer were transferred onto the surface of the magnetic layer to form alarge number of recesses on the surface of the magnetic layer. Thenumber of recesses having a depth of 20% or more of the averagethickness of the magnetic layer was set to 15 per 1600 μm².

<Cutting Step>

The magnetic recording medium obtained as described above was cut into awidth of ½ inches (12.65 mm). As a result, the target long magneticrecording medium (average thickness 5.6 μm) was obtained. Note that theBET specific surface area of the obtained magnetic recording medium was4 m²/g in a state where the lubricant has been removed from the magneticrecording medium and the magnetic recording medium has been dried.

Examples 2 and 13

Magnetic recording media as Examples 2 and 13 were obtained in a similarmanner to Example 1 described above except that the drying conditionswere adjusted in the application step, and the squareness ratio S1 ofthe magnetic recording medium in the thickness direction (perpendiculardirection) and the squareness ratio S2 thereof in the longitudinaldirection were set to the values illustrated in Table 2.

Examples 3 and 4

The particle diameter included in the back layer was changed such thatthe number of recesses having a depth of 20% or more of the averagethickness of the magnetic layer was 13 per 1600 μm². Furthermore,magnetic recording media as Examples 3 and 4 were obtained in a similarmanner to Example 1 described above except that the drying conditionswere adjusted in the application step, and the squareness ratio S1 ofthe magnetic recording medium in the thickness direction (perpendiculardirection) and the squareness ratio S2 thereof in the longitudinaldirection were set to the values illustrated in Table 2.

Example 5

In the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 2. Moreover, the heating conditions were adjusted in the transferstep, and the BET specific surface area was set to 4.5 m²/g. A magneticrecording medium as Example 5 was obtained in a similar manner toExample 1 described above except for these.

Example 6

The particle diameter included in the back layer was changed such thatthe number of recesses having a depth of 20% or more of the averagethickness of the magnetic layer was 18 per 1600 μm². Furthermore, in theapplication step, drying conditions were adjusted, and the squarenessratio S1 of the magnetic recording medium in the thickness direction(perpendicular direction) and the squareness ratio S2 thereof in thelongitudinal direction were set to the values illustrated in Table 2.Moreover, the heating conditions were adjusted in the transfer step, andthe BET specific surface area was set to 5.0 m²/g. A magnetic recordingmedium as Example 5 was obtained in a similar manner to Example 1described above except for these.

Example 7

In the step of preparing the magnetic layer forming coating material,powder of strontium ferrite particles (hexagonal plate shape, averageaspect ratio 3.0, average particle size 21.3 nm, particle volume 2000nm³) was used as the magnetic powder. Furthermore, in the applicationstep, drying conditions were adjusted, and the squareness ratio S1 ofthe magnetic recording medium in the thickness direction (perpendiculardirection) and the squareness ratio S2 thereof in the longitudinaldirection were set to the values illustrated in Table 2. A magneticrecording medium as Example 7 was obtained in a similar manner toExample 1 described above except for these.

Example 8

In the step of preparing the magnetic layer forming coating material,powder of ε iron oxide particles (spherical shape, average aspect ratio1.1, average particle size 16 nm, particle volume 2150 nm³) was used asthe magnetic powder. Furthermore, in the application step, dryingconditions were adjusted, and the squareness ratio S1 of the magneticrecording medium in the thickness direction (perpendicular direction)and the squareness ratio S2 thereof in the longitudinal direction wereset to the values illustrated in Table 2. A magnetic recording medium asExample 8 was obtained in a similar manner to Example 1 described aboveexcept for these.

Example 9

In the step of preparing the magnetic layer forming coating material,cobalt ferrite powder (cubic shape, average aspect ratio 1.7, averageparticle size 18.5 nm, particle volume 2200 nm³) was used as themagnetic powder. Furthermore, in the application step, drying conditionswere adjusted, and the squareness ratio S1 of the magnetic recordingmedium in the thickness direction (perpendicular direction) and thesquareness ratio S2 thereof in the longitudinal direction were set tothe values illustrated in Table 2. A magnetic recording medium asExample 9 was obtained in a similar manner to Example 1 described aboveexcept for these.

Example 10

In the back layer forming coating material, the blending amount ofcarbon black powder having a small particle diameter (average particlediameter (D50) 20 nm) was set to 80 parts by mass, and the blendingamount of carbon black powder having a large particle diameter (averageparticle diameter (D50) 270 nm) was set to 20 parts by mass.Furthermore, the heating conditions were adjusted in the transfer step,the pore volume was set to 0.023 cm³/g, and the pore diameter of themaximum pore volume at the time of desorption was set to 9 nm. Moreover,in the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 2. A magnetic recording medium as Example 10 was obtained in asimilar manner to Example 1 described above except for these.

Example 11

The heating conditions were adjusted in the transfer step, and the porediameter of the maximum pore volume at the time of desorption was set to10 nm. Furthermore, in the application step, drying conditions wereadjusted, and the squareness ratio S1 of the magnetic recording mediumin the thickness direction (perpendicular direction) and the squarenessratio S2 thereof in the longitudinal direction were set to the valuesillustrated in Table 2. A magnetic recording medium as Example 11 wasobtained in a similar manner to Example 1 described above except forthese.

Example 12

In forming the back layer forming coating material, 70 parts by mass ofcarbon black powder having a small particle diameter (average particlediameter (D50) 50 nm) was blended instead of carbon black powder havinga small particle diameter (average particle diameter (D50) 20 nm), andthe blending amount of carbon black powder having a large particlediameter (average particle diameter (D50) 270 nm) was set to 30 parts bymass. Moreover, the heating conditions were adjusted in the transferstep. As a result, the pore diameter of the maximum pore volume at thetime of desorption was set to 12 nm, the number of recesses having adepth of 20% or more of the average thickness of the magnetic layer wasset to 20 per 1600 μm², and the kurtosis of the surface of the backlayer was set to 2.6. Furthermore, in the application step, dryingconditions were adjusted, and the squareness ratio S1 of the magneticrecording medium in the thickness direction (perpendicular direction)and the squareness ratio S2 thereof in the longitudinal direction wereset to the values illustrated in Table 2. Moreover, the heatingconditions were adjusted in the transfer step, and the BET specificsurface area was set to 6.0 m²/g. A magnetic recording medium as Example12 was obtained in a similar manner to Example 1 described above exceptfor these.

Example 14

In the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 2. Moreover, the heating conditions were adjusted in the transferstep, and the BET specific surface area was set to 3.9 m²/g. A magneticrecording medium as Example 14 was obtained in a similar manner toExample 1 described above except for these.

Example 15

In the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 2. Moreover, the heating conditions were adjusted in the transferstep, and the BET specific surface area was set to 3.8 m²/g. A magneticrecording medium as Example 15 was obtained in a similar manner toExample 1 described above except for these.

Example 16

A magnetic recording medium as Example 16 was obtained in a similarmanner to Example 1 described above except that powder of hexagonalplate-shaped barium ferrite particles having an average aspect ratio of2.5, an average particle size of 19.0 nm, and an average particle volumeof 1600 nm³ was used as the magnetic powder in the step of preparing themagnetic layer forming coating material. Note that the pore diameter ofthe maximum pore volume at the time of desorption was set to 7 nm, andthe number of recesses having a depth of 20% or more of the averagethickness of the magnetic layer was set to 16 per 1600 μm².

Example 17

A magnetic recording medium as Example 21 was obtained in a similarmanner to Example 1 described above except that powder of hexagonalplate-shaped barium ferrite particles having an average aspect ratio of2.3, an average particle size of 17.0 nm, and an average particle volumeof 1300 nm³ was used as the magnetic powder in the step of preparing themagnetic layer forming coating material. Note that the pore diameter ofthe maximum pore volume at the time of desorption was set to 6 nm, andthe number of recesses having a depth of 20% or more of the averagethickness of the magnetic layer was set to 17 per 1600 μm².

Example 18

The average thickness of the magnetic layer was set to 60 nm, and theaverage thickness of the magnetic recording medium was set to 4.3 μm.Furthermore, in the application step, drying conditions were adjusted,and the squareness ratio S1 of the magnetic recording medium in thethickness direction (perpendicular direction) and the squareness ratioS2 thereof in the longitudinal direction were set to the valuesillustrated in Table 2. A magnetic recording medium as Example 18 wasobtained in a similar manner to Example 1 described above except forthese. Note that the BET specific surface area was 3.9 m²/g.

Example 19

The average thickness of the magnetic layer was set to 40 nm, and theaverage thickness of the magnetic recording medium was set to 4.3 μm.Furthermore, in the application step, drying conditions were adjusted,and the squareness ratio S1 of the magnetic recording medium in thethickness direction (perpendicular direction) and the squareness ratioS2 thereof in the longitudinal direction were set to the valuesillustrated in Table 2. A magnetic recording medium as Example 19 wasobtained in a similar manner to Example 1 described above except forthese. Note that the BET specific surface area was 3.8 m²/g.

Example 20

In the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 2. Moreover, the heating conditions were adjusted in the transferstep, the number of recesses having a depth of 20% or more of theaverage thickness of the magnetic layer was set to 11 per 1600 μm², andthe BET specific surface area was set to 3.5 m²/g. A magnetic recordingmedium as Example 20 was obtained in a similar manner to Example 1described above except for these.

Example 21

In forming the back layer forming coating material, 60 parts by mass ofcarbon black powder having a small particle diameter (average particlediameter (D50) 50 nm) was blended instead of carbon black powder havinga small particle diameter (average particle diameter (D50) 20 nm), andthe blending amount of carbon black powder having a large particlediameter (average particle diameter (D50) 270 nm) was set to 40 parts bymass. Moreover, the heating conditions were adjusted in the transferstep. As a result, the number of recesses having a depth of 20% or moreof the average thickness of the magnetic layer was set to 22 per 1600μm², and the kurtosis of the surface of the back layer was set to 3.5.Furthermore, in the application step, drying conditions were adjusted,and the squareness ratio S1 of the magnetic recording medium in thethickness direction (perpendicular direction) and the squareness ratioS2 thereof in the longitudinal direction were set to the valuesillustrated in Table 2. A magnetic recording medium as Example 21 wasobtained in a similar manner to Example 1 described above except forthese.

Example 22

In the back layer forming coating material, the blending amount ofcarbon black powder having a small particle diameter (average particlediameter (D50) 20 nm) was set to 70 parts by mass, and the blendingamount of carbon black powder having a large particle diameter (averageparticle diameter (D50) 270 nm) was set to 30 parts by mass. Moreover,the heating conditions were adjusted in the transfer step. As a result,the number of recesses having a depth of 20% or more of the averagethickness of the magnetic layer was set to 160 per 1600 μm², and thekurtosis of the surface of the back layer was set to 3.0. Furthermore,in the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 2. A magnetic recording medium as Example 22 was obtained in asimilar manner to Example 1 described above except for these.

Comparative Example 1

In the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 3. Moreover, the heating conditions were adjusted in the transferstep, the number of recesses having a depth of 20% or more of theaverage thickness of the magnetic layer was set to 12 per 1600 μm², andthe BET specific surface area was set to 3.0 m²/g. A magnetic recordingmedium as Comparative Example 1 was obtained in a similar manner toExample 1 described above except for these.

Comparative Example 2

In the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 3. Moreover, the heating conditions were adjusted in the transferstep, the number of recesses having a depth of 20% or more of theaverage thickness of the magnetic layer was set to 14 per 1600 μm², andthe BET specific surface area was set to 2.0 m²/g. A magnetic recordingmedium as Comparative Example 2 was obtained in a similar manner toExample 1 described above except for these.

Comparative Example 3

In forming the back layer forming coating material, 80 parts by mass ofcarbon black powder having a small particle diameter (average particlediameter (D50) 50 nm) was blended instead of carbon black powder havinga small particle diameter (average particle diameter (D50) 20 nm), andthe blending amount of carbon black powder having a large particlediameter (average particle diameter (D50) 270 nm) was set to 20 parts bymass. Moreover, the heating conditions were adjusted in the transferstep. As a result, the number of recesses having a depth of 20% or moreof the average thickness of the magnetic layer was set to 16 per 1600μm², and the kurtosis of the surface of the back layer was set to 3.0.Furthermore, in the application step, drying conditions were adjusted,and the squareness ratio S1 of the magnetic recording medium in thethickness direction (perpendicular direction) and the squareness ratioS2 thereof in the longitudinal direction were set to the valuesillustrated in Table 3. Moreover, the heating conditions were adjustedin the transfer step, the pore volume was set to 0.018 cm³/g, and theBET specific surface area was set to 3.0 m²/g. A magnetic recordingmedium as Comparative Example 3 was obtained in a similar manner toExample 1 described above except for these.

Comparative Example 4

In forming the back layer forming coating material, 90 parts by mass ofcarbon black powder having a small particle diameter (average particlediameter (D50) 50 nm) was blended instead of carbon black powder havinga small particle diameter (average particle diameter (D50) 20 nm), andthe blending amount of carbon black powder having a large particlediameter (average particle diameter (D50) 270 nm) was set to 10 parts bymass. Moreover, the heating conditions were adjusted in the transferstep. As a result, the number of recesses having a depth of 20% or moreof the average thickness of the magnetic layer was set to 13 per 1600μm². Furthermore, in the application step, drying conditions wereadjusted, and the squareness ratio S1 of the magnetic recording mediumin the thickness direction (perpendicular direction) and the squarenessratio S2 thereof in the longitudinal direction were set to the valuesillustrated in Table 3. Moreover, the heating conditions were adjustedin the transfer step, the pore volume was 0.015 cm³/g, and the BETspecific surface area was 2.5 m²/g. A magnetic recording medium asComparative Example 4 was obtained in a similar manner to Example 1described above except for these.

Comparative Example 5

In forming the back layer forming coating material, 100 parts by mass ofonly carbon black powder having a small particle diameter (averageparticle diameter (D50) 50 nm) was blended instead of carbon blackpowder having a small particle diameter (average particle diameter (D50)20 nm) as the carbon black powder. Moreover, the heating conditions wereadjusted in the transfer step. As a result, the number of recesseshaving a depth of 20% or more of the average thickness of the magneticlayer was set to 12 per 1600 μm², and the kurtosis of the surface of theback layer was set to 1.6. Furthermore, in the application step, dryingconditions were adjusted, and the squareness ratio S1 of the magneticrecording medium in the thickness direction (perpendicular direction)and the squareness ratio S2 thereof in the longitudinal direction wereset to the values illustrated in Table 3. Moreover, the heatingconditions were adjusted in the transfer step, the pore volume was setto 0.015 cm³/g, the pore diameter of the maximum pore volume at the timeof desorption was set to 5 nm, and the BET specific surface area was setto 2.0 m²/g. A magnetic recording medium as Comparative Example 5 wasobtained in a similar manner to Example 1 described above except forthese.

Comparative Example 6

The heating conditions were adjusted in the transfer step, and thekurtosis of the surface of the back layer was set to 1.8. A magneticrecording medium as Comparative Example 6 was obtained in a similarmanner to Example 1 described above except for this.

Comparative Example 7

The heating conditions were adjusted in the transfer step, the number ofrecesses having a depth of 20% or more of the average thickness of themagnetic layer was set to 9 per 1600 μm², and the kurtosis on thesurface of the back layer was set to 2.4. A magnetic recording medium asComparative Example 7 was obtained in a similar manner to Example 1described above except for these.

Comparative Example 8

In the back layer forming coating material, the blending amount ofcarbon black powder having a small particle diameter (average particlediameter (D50) 20 nm) was set to 70 parts by mass, and the blendingamount of carbon black powder having a large particle diameter (averageparticle diameter (D50) 270 nm) was set to 30 parts by mass. Moreover,the heating conditions were adjusted in the transfer step. As a result,the number of recesses having a depth of 20% or more of the averagethickness of the magnetic layer was set to 210 per 1600 μm², and thekurtosis of the surface of the back layer was set to 4.5. Furthermore,in the application step, drying conditions were adjusted, and thesquareness ratio S1 of the magnetic recording medium in the thicknessdirection (perpendicular direction) and the squareness ratio S2 thereofin the longitudinal direction were set to the values illustrated inTable 2. A magnetic recording medium as Comparative Example 8 wasobtained in a similar manner to Example 1 described above except forthese.

[Evaluation]

For the magnetic recording media of Examples 1 to 22 and ComparativeExamples 1 to 8 obtained as described above, the following evaluationwas performed in addition to the above-described friction coefficientratio (μ_(B)/μ_(A)) and friction coefficient ratio(μ_(C(1000))/μ_(C(5))).

(SNR)

Using a ½ inch tape traveling device (manufactured by MountainEngineering II, MTS Transport) equipped with a recording/reproducinghead and a recording/reproducing amplifier, the electromagneticconversion characteristics (SNR) of each of the magnetic recording mediawere measured in an environment of 25° C. A ring head having a gaplength of 0.2 μm was used as the recording head, and a GMR head having ashield-to-shield distance of 0.1 μm was used as the reproducing head. Arelative speed, a recording clock frequency, and a recording track widthwere set to 6 m/s, 160 MHz, and 2.0 μm, respectively. Furthermore, theSNR was calculated on the basis of a method described in the followingdocument. The results are illustrated in Table 2 as relative values withthe SNR of Example 1 as 1 dB.

Y Okazaki: “An Error Rate Emulation System.”, IEEE Trans. Man., 31, pp.3093-3095 (1995)

(Winding Deviation)

In each of the magnetic recording media wound on a spindle after theabove-described cutting step, the degree of winding deviation in thewidth direction of the magnetic recording medium was evaluated.

A tape cartridge was caused to travel over one round trip full lengthwith a drive. Thereafter, a drop test was performed again over the fulllength according to the general rules of JIS Z0200 packaging cargoperformance test method. After the drop test, a sample that could besubjected to recording operation and reproducing operation again overthe full length was judged to have good winding deviation, and a samplethat caused an error in the middle in at least one of the recordingoperation and the reproducing operation was judged to have poor windingdeviation. A sample in which winding deviation has occurred causes poortraveling because an edge of the tape is pressed against a reel flangeby an impact at the time of dropping, and edge break occurs at a portionwhere the deviation has occurred.

Table 2 summarizes the configurations and evaluation results of themagnetic recording media in Examples and Comparative Examples.

TABLE 2 Square- ness ratio in Pore perpen- diameter dicular in directionSquare- maximum Friction (no ness pore coeff- Average demag- ratio involume at BET icient thickness Number netizing longitu- the timespecific Friction ratio Aver- Average of Average Ra of of 20% fielddinal Pore of surface coefficient μC SNR Tape age Particle particlemagnetic thickness magnetic Sku of recesses/ correction) directionvolume desorption area Magnetic ratio (1000)/ charac- winding aspectvolume size layer of tape layer back 1600 % % cm³/g nm m²/g powder μB/μAμC(5) teristics deviation Shape ratio nm³ nm nm μm nm surface μm² Ex- 6535 0.020 8 4 BaFe₁₂O₁₉ 1.2 1.2 1 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.315 am- shape or less ple 1 Ex- 66 30 0.020 8 4 BaFe₁₂O₁₉ 1.2 1.2 1.2Good Plate 2.8 1950 20.3 80 5.6 2.5 2.3 15 am- shape or less ple 2 Ex-71 29 0.020 8 4 BaFe₁₂O₁₉ 1.2 1.3 1.4 Good Plate 2.8 1950 20.3 80 5.62.5 2.3 13 am- shape or less ple 3 Ex- 70 25 0.020 8 4 BaFe₁₂O₁₉ 1.2 1.21.5 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.3 13 am- shape or less ple 4Ex- 66 30 0.020 8 4.5 BaFe₁₂O₁₉ 1.2 1.2 1.1 Good Plate 2.8 1950 20.3 805.6 2.5 2.2 15 am- shape or less ple 5 Ex- 66 30 0.020 8 5 SrFe₁₂O₁₉ 1.21.3 1.1 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.3 18 am- shape or less ple6 Ex- 66 30 0.020 8 4 SrFe₁₂O₁₉ 1.2 1.2 1.1 Good Plate 3.0 2000 21.3 805.6 2.5 2.3 15 am- shape or less ple 7 Ex- 66 30 0.020 8 4 ε iron 1.21.3 1.2 Good Spherical 1.1 2150 16 80 5.6 2.5 2.3 15 am- oxide shape orless ple 8 Ex- 66 30 0.020 8 4 Co-iron 1.2 1.2 1.3 Good Cubic 1.7 220018.5 80 5.6 2.5 2.3 15 am- oxide shape or less ple 9 Ex- 66 30 0.023 9 4BaFe₁₂O₁₉ 1.2 1.1 1.2 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.3 15 am-shape or less ple 10 Ex- 66 30 0.020 10 4 BaFe₁₂O₁₉ 1.2 1 1.2 Good Plate2.8 1950 20.3 80 5.6 2.5 2.3 15 am- shape or less ple 11 Ex- 66 31 0.02012 6 BaFe₁₂O₁₉ 2.1 1.9 1.1 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.6 20am- shape or less ple 12 Ex- 75 23 0.020 8 4 BaFe₁₂O₁₉ 1.2 1.2 1.6 GoodPlate 2.8 1950 20.3 80 5.6 2.5 2.3 15 am- shape or less ple 13 Ex- 80 210.020 8 3.9 BaFe₁₂O₁₉ 1.2 1.2 1.9 Good Plate 2.8 1950 20.3 80 5.6 2.52.3 15 am- shape or less ple 14 Ex- 85 18 0.020 8 3.8 BaFe₁₂O₁₉ 1.2 1.22.2 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.3 15 am- shape or less ple 15Ex- 65 35 0.020 7 4 BaFe₁₂O₁₉ 1.2 1.2 1.8 Good Plate 2.5 1600 19 80 5.62.5 2.3 16 am- shape or less ple 16 Ex- 65 35 0.020 6 4 BaFe₁₂O₁₉ 1.21.4 2.2 Good Plate 2.3 1300 17 80 5.6 2.5 2.3 17 am- shape or less ple17 Ex- 75 23 0.020 8 3.9 BaFe₁₂O₁₉ 1.4 1.5 1.6 Good Plate 2.8 1950 20.360 4.3 2.5 2.3 15 am- shape or less ple 18 Ex- 80 20 0.020 8 3.8BaFe₁₂O₁₉ 1.6 1.6 1.6 Good Plate 2.8 1950 20.3 40 4.3 2.5 2.3 15 am-shape or less ple 19 Ex- 66 30 0.020 8 3.5 BaFe₁₂O₂₀ 1.2 1.3 1.1 GoodPlate 2.8 1950 20.3 80 5.6 2.5 2.3 11 am- shape or less ple 20 Ex- 66 300.020 8 4 BaFe₁₂O₂₀ 1.2 1.2 1.2 Good Plate 2.8 1950 20.3 80 5.6 2.5 3.522 am- shape or less ple 21 Ex- 66 30 0.020 8 4 BaFe₁₂O₂₁ 1.2 1.2 1.2Good Plate 2.8 1950 20.3 80 5.6 2.5 3 160 am- shape or less ple 22 Com-66 31 0.020 8 3 BaFe₁₂O₁₉ 2.2 2.3 1 Good Plate 2.8 1950 20.3 80 5.6 2.52.3 12 para- shape or less tive Ex- am- ple 1 Com- 66 31 0.020 8 2BaFe₁₂O₁₉ 2.3 2.5 1.1 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.3 14 para-shape or less tive Ex- am- ple 2 Com- 66 31 0.018 8 3 BaFe₁₂O₁₉ 2.2 2 1Good Plate 2.8 1950 20.3 80 5.6 2.5 3 16 para- shape or less tive Ex-am- ple 3 Com- 66 31 0.015 8 2.5 BaFe₁₂O₁₉ 2.2 2.2 1.1 Good Plate 2.81950 20.3 80 5.6 2.5 2.3 13 para- shape or less tive Ex- am- ple 4 Com-66 31 0.015 5 2 BaFe₁₂O₁₉ 2.2 2.3 1 Good Plate 2.8 1950 20.3 80 5.6 2.51.6 12 para- shape or less tive Ex- am- ple 5 Com- 65 35 0.020 8 4BaFe₁₂O₁₉ 1.2 2.1 1.0 Poor Plate 2.8 1950 20.3 80 5.6 2.5 1.8 15 para-shape or less tive Ex- am- ple 6 Com- 65 35 0.020 8 4 BaFe₁₂O₁₉ 1.2 2.10.5 Good Plate 2.8 1950 20.3 80 5.6 2.5 2.4 9 para- shape or less tiveEx- am- ple 7 Com- 66 31 0.020 8 4 BaFe₁₂O₁₉ 1.2 1.2 0.3 Good Plate 2.81950 20.3 80 5.6 2.5 4.5 210 para- shape or less tive Ex- am- ple 8

As illustrated in Table 2, in Examples 1 to 22, since the BET specificsurface area of the entire magnetic recording medium 10 in a state wherethe lubricant had been removed from the magnetic recording medium 10 andthe magnetic recording medium 10 had been dried was 3.5 m²/g or more,even after repeated recording or reproduction was performed, thelubricant was stably supplied to an interface between the magneticrecording medium and the magnetic head, and an increase in the frictioncoefficient ratio could be suppressed. Meanwhile, in ComparativeExamples 1 to 5, since the BET specific surface area of the entiremagnetic recording medium in a state where the lubricant had beenremoved was less than 3.5 m²/g, the friction coefficient ratio increasedafter repeated recording or reproduction was performed.

Furthermore, in Examples 1 to 22, since the kurtosis of the surface 14Sof the back layer 14 was 2.0 or more and 4.0 or less, an appropriatefrictional force was generated also at an interface between the surface13S and the surface 14S where the lubricant was likely to appear, andwinding deviation of the magnetic recording medium could be prevented.Meanwhile, in Comparative Example 6, since the kurtosis of the surface14S of the back layer 14 was 1.8, winding deviation of the magneticrecording medium occurred.

Furthermore, in Examples 1 to 22, since the squareness ratio S1 of themagnetic recording medium in a perpendicular direction (thicknessdirection) thereof was 65% or more, and the recesses having a depth of20% or more of the average thickness of the magnetic layer were formedat a ratio of 10 or more and 200 or less per 1600 μm², a good SNR wasobtained. It is considered that this is because air trapped while themagnetic recording medium is traveling can be released from theinterface between the surface of the magnetic layer and the head, andcontact between the surface of the magnetic layer and the head and canbe kept well. Meanwhile, in Comparative Example 7, since the recesseshaving a depth of 20% or more of the average thickness of the magneticlayer were formed at a ratio of 9 per 1600 μm², an SNR deteriorated. Itis considered that this is because air trapped while the magneticrecording medium was traveling could not be released sufficiently fromthe interface between the surface of the magnetic layer and the head, anair layer was generated between the surface of the magnetic layer andthe head, and the surface of the magnetic layer was not sufficiently incontact with the head. Furthermore, in Comparative Example 8, since therecesses 13A having a depth of 20% or more of the average thickness ofthe magnetic layer 13 were formed at a ratio of 210 per 1600 μm², an SNRalso deteriorated. It is considered that this is because the contactarea between the surface of the magnetic layer and the head decreaseddue to the too high surface density of the recesses.

Although the present disclosure has been specifically described withreference to the embodiment and Modifications thereof, the presentdisclosure is not limited to the above-described embodiment and thelike, and various modifications can be made.

For example, the configurations, the methods, the steps, the shapes, thematerials, the numerical values, and the like exemplified in theembodiment described above and Modifications thereof are only examples,and a configuration, a method, a step, a shape, a material, a numericalvalue, and the like different therefrom may be used as necessary.

Specifically, the magnetic recording medium of the present disclosuremay include components other than the substrate, the base layer, themagnetic layer, the back layer, and the barrier layer. Furthermore, thechemical formulas of the compounds and the like are representative andare not limited to the described valences and the like as long as thecompounds have common names of the same compound.

Furthermore, the configurations, the methods, the steps, the shapes, thematerials, the numerical values, and the like in the embodimentdescribed above and Modifications thereof can be combined to each otheras long as not departing from the gist of the present disclosure.

Furthermore, within the numerical range described step by step here, anupper limit value or a lower limit value of a numerical range in onestage may be replaced with an upper limit value or a lower limit valueof a numerical range in another stage. The materials exemplified herecan be used singly or in combination of two or more thereof unlessotherwise specified.

As described above, the magnetic recording medium according to anembodiment of the present disclosure can exhibit good travelingperformance during use.

Note that the effect of the present disclosure is not limited thereto,and may be any effect described here. Furthermore, the presenttechnology can take the following configurations.

(1)

A tape-shaped magnetic recording medium including:

a substrate;

a base layer disposed on the substrate;

a magnetic layer disposed on the base layer; and

a back layer disposed on a side of the substrate opposite to the baselayer, in which

the substrate includes a polyester as a main component,

a surface of the back layer opposite to the substrate has a kurtosis of2.0 or more,

on a surface of the magnetic layer, recesses having a depth of 20% ormore of the average thickness of the magnetic layer are formed at aratio of 10 or more and 200 or less per 1600 μm²,

the surface of the magnetic layer has arithmetic average roughness Ra of2.5 nm or less,

the base layer and the magnetic layer each include a lubricant,

the entire magnetic recording medium has a BET specific surface area of3.5 m²/g or more in a state where the lubricant has been removed fromthe magnetic recording medium and the magnetic recording medium has beendried,

a squareness ratio in a perpendicular direction is 65% or more,

the magnetic layer has an average thickness of 90 nm or less, and

the magnetic recording medium has an average thickness of 5.6 μm orless.

(2)

The magnetic recording medium according to (1) described above, in whichthe kurtosis of the surface of the back layer is 3.0 or more.

(3)

The magnetic recording medium according to (1) described above, in whichthe kurtosis of the surface of the back layer is 3.5 or more.

(4)

The magnetic recording medium according to any one of (1) to (3)described above, in which

on a surface of the magnetic layer, the recesses are formed at a ratioof 15 or more and 200 or less per 1600 μm².

(5)

The magnetic recording medium according to any one of (1) to (3)described above, in which

on a surface of the magnetic layer, the recesses are formed at a ratioof 20 or more and 200 or less per 1600 μm².

(6)

The magnetic recording medium according to any one of (1) to (5)described above, in which

the entire magnetic recording medium has an average pore diameter of 6nm or more and 12 nm or less, the average pore diameter being determinedby a BJH method.

(7)

The magnetic recording medium according to any one of (1) to (6)described above, in which

in a case where a coefficient of dynamic friction between the surfaceand a magnetic head is represented by μ_(A) when a tension of 0.4 N isapplied to the magnetic recording medium, and a coefficient of dynamicfriction between the surface and the magnetic head is represented by μBwhen a tension of 1.2 N is applied to the magnetic recording medium, afriction coefficient ratio μ_(B)/μ_(A) A is 1.2 or more and 2.1 or less.

(8)

The magnetic recording medium according to any one of (1) to (7)described above, in which

in a case where a fifth coefficient of dynamic friction between thesurface and a magnetic head from start of travel of the magneticrecording medium is represented by μ_(C)(5) when a tension of 0.6 N isapplied to the magnetic recording medium, and a 1000th coefficient ofdynamic friction between the surface and the magnetic head from thestart of travel of the magnetic recording medium is represented byμC(1000) when a tension of 0.6 N is applied to the magnetic recordingmedium, a friction coefficient ratio μC(1000)/μC(5) is 1.2 or more and1.9 or less.

(9)

The magnetic recording medium according to any one of (1) to (8)described above, in which

the magnetic layer includes magnetic powder, and

the magnetic powder has an average aspect ratio of 1.1 or more and 3.0or less.

(10)

The magnetic recording medium according to any one of (1) to (9)described above, in which

the magnetic layer includes magnetic powder, and

the magnetic powder includes hexagonal ferrite including at least one ofbarium (Ba) and strontium (Sr), ε iron oxide, or cobalt (Co)-containingspinel type ferrite.

(11)

The magnetic recording medium according to any one of (1) to (10)described above, in which

the magnetic layer includes magnetic powder, and

the magnetic powder has an average particle size of 25 nm or less.

(12)

The magnetic recording medium according to any one of (1) to (10)described above, in which

the lubricant includes at least one of a compound represented by thefollowing general formula <1> and a compound represented by thefollowing general formula <2>, and at least one of a compoundrepresented by the following general formula <3> and a compoundrepresented by the following general formula <4>.CH₃(CH₂)_(k)COOH  <1>

(Provided that in general formula <1>, k is an integer selected from arange of 14 or more and 22 or less.)CH₃(CH₂)_(n)CH═CH(CH₂)_(m)COOH  <2>

(Provided that in general formula <2>, the sum of n and m is an integerselected from a range of 12 or more and 20 or less, more preferably arange of 14 or more and 18 or less.)CH₃(CH₂)_(p)COO(CH₂)_(q)CH₃  <3>

(Provided that in general formula <3>, p is an integer selected from arange of 14 or more and 22 or less, more preferably a range of 14 ormore and 18 or less, and q is an integer selected from a range of 2 ormore and 5 or less, more preferably a range of 2 or more and 4 or less.)CH₃(CH₂)_(p)COO—(CH₂)_(q)CH(CH₃)₂  <4>

(Provided that in the general formula <2>, p is an integer selected froma range of 14 or more and 22 or less, and q is an integer selected froma range of 1 or more and 3 or less.)

(13)

The magnetic recording medium according to any one of (1) to (12)described above, in which

the base layer has a large number of holes, and

the recesses of the magnetic layer are connected to the holes of thebase layer.

(14)

A magnetic recording/reproducing device including:

a feeding unit that can sequentially feed out a tape-shaped magneticrecording medium;

a winding unit that can wind up the magnetic recording medium fed outfrom the feeding unit; and

a magnetic head that can write information on the magnetic recordingmedium and can read out information from the magnetic recording mediumwhile being in contact with the magnetic recording medium traveling fromthe feeding unit toward the winding unit, in which

the magnetic recording medium includes:

a substrate;

a base layer disposed on the substrate;

a magnetic layer disposed on the base layer; and

a back layer disposed on a side of the substrate opposite to the baselayer,

the substrate includes a polyester as a main component,

a surface of the back layer opposite to the substrate has a kurtosis of2.0 or more,

on a surface of the magnetic layer, recesses having a depth of 20% ormore of the average thickness of the magnetic layer are formed at aratio of 10 or more and 200 or less per 1600 μm²,

a second surface of the magnetic layer has arithmetic average roughnessRa of 2.5 nm or less,

the base layer and the magnetic layer each include a lubricant,

the entire magnetic recording medium has a BET specific surface area of3.5 m²/g or more in a state where the lubricant has been removed fromthe magnetic recording medium and the magnetic recording medium has beendried,

a squareness ratio in a perpendicular direction is 65% or more,

the magnetic layer has an average thickness of 90 nm or less, and

the magnetic recording medium has an average thickness of 5.6 μm orless.

(15)

The magnetic recording/reproducing device according to (14) describedabove, in which

a tension applied to the magnetic recording medium in a longitudinaldirection thereof can be adjusted.

(16)

A magnetic recording medium cartridge including:

the tape-shaped magnetic recording medium according to any one of (1) to(13) described above; and

a casing that houses the magnetic recording medium.

This application claims the benefit of priority based on Japanese PatentApplication No. 2019-150674 filed on Aug. 20, 2019, the entire contentsof which are incorporated herein by reference.

A person skilled in the art can conceive of various modifications,combinations, sub-combinations, and changes, in accordance with designrequirements and other factors. It is understood that thesemodifications, combinations, sub-combinations, and changes are includedin the appended claims and the scope of equivalents thereof.

The invention claimed is:
 1. A magnetic recording medium comprising: asubstrate; a base layer disposed on the substrate; a magnetic layerdisposed on the base layer; and a back layer disposed on a side of thesubstrate opposite to the base layer, wherein the substrate includes apolyester as a main component, a surface of the back layer opposite tothe substrate has a kurtosis of 2.0 or more, on a surface of themagnetic layer, recesses having a depth of 20% or more of the averagethickness of the magnetic layer are formed at a ratio of 10 or more and200 or less per 1600 μm², the surface of the magnetic layer hasarithmetic average roughness Ra of 2.5 nm or less, the base layer andthe magnetic layer each include a lubricant, the entire magneticrecording medium has a BET specific surface area of 3.5 m²/g or more ina state where the lubricant has been removed from the magnetic recordingmedium and the magnetic recording medium has been dried, a squarenessratio in a perpendicular direction is 65% or more, the magnetic layerhas an average thickness of 90 nm or less, the magnetic recording mediumhas an average thickness of 5.6 μm or less, and the magnetic recordingmedium is tape-shaped.
 2. The magnetic recording medium according toclaim 1, wherein the kurtosis of the surface of the back layer is 3.0 ormore.
 3. The magnetic recording medium according to claim 1, wherein thekurtosis of the surface of the back layer is 3.5 or more.
 4. Themagnetic recording medium according to claim 1, wherein on a surface ofthe magnetic layer, the recesses are formed at a ratio of 15 or more and200 or less per 1600 μm².
 5. The magnetic recording medium according toclaim 1, wherein on a surface of the magnetic layer, the recesses areformed at a ratio of 20 or more and 200 or less per 1600 μm².
 6. Themagnetic recording medium according to claim 1, wherein the entiremagnetic recording medium has an average pore diameter of 6 nm or moreand 12 nm or less, the average pore diameter being determined by a BJHmethod.
 7. The magnetic recording medium according to claim 1, whereinin a case where a coefficient of dynamic friction between the surfaceand a magnetic head is represented by μA when a tension of 0.4 N isapplied to the magnetic recording medium, and a coefficient of dynamicfriction between the surface and the magnetic head is represented by μBwhen a tension of 1.2 N is applied to the magnetic recording medium, afriction coefficient ratio μB/μA is 1.2 or more and 2.1 or less.
 8. Themagnetic recording medium according to claim 1, wherein in a case wherea fifth coefficient of dynamic friction between the surface and amagnetic head from start of travel of the magnetic recording medium isrepresented by μC(5) when a tension of 0.6 N is applied to the magneticrecording medium, and a 1000th coefficient of dynamic friction betweenthe surface and the magnetic head from the start of travel of themagnetic recording medium is represented by μC(1000) when a tension of0.6 N is applied to the magnetic recording medium, a frictioncoefficient ratio μC(1000)/μC(5) is 1.2 or more and 1.9 or less.
 9. Themagnetic recording medium according to claim 1, wherein the magneticlayer includes magnetic powder, and the magnetic powder has an averageaspect ratio of 1.1 or more and 3.0 or less.
 10. The magnetic recordingmedium according to claim 1, wherein the magnetic layer includesmagnetic powder, and the magnetic powder includes hexagonal ferriteincluding at least one of barium (Ba) and strontium (Sr), ε iron oxide,or cobalt (Co)-containing spinel type ferrite.
 11. The magneticrecording medium according to claim 1, wherein the magnetic layerincludes magnetic powder, and the magnetic powder has an averageparticle size of 25 nm or less.
 12. The magnetic recording mediumaccording to claim 1, wherein the lubricant includes at least one of acompound represented by the following general formula <1> and a compoundrepresented by the following general formula <2>, and at least one of acompound represented by the following general formula <3> and a compoundrepresented by the following general formula <4>CH₃(CH₂)_(k)COOH  <1> Provided that in general formula <1>, k is aninteger selected from a range of 14 or more and 22 or lessCH₃(CH₂)_(n)CH═CH(CH₂)_(m)COOH  <2> Provided that in general formula<2>, a sum of n and m is an integer selected from a range of 12 or moreand 20 or lessCH₃(CH₂)_(p)COO(CH₂)_(q)CH₃  <3> Provided that in general formula <3>, pis an integer selected from a range of 14 or more and 22 or less and qis an integer selected from a range of 2 or more and 5 or lessCH₃(CH₂)_(p)COO—(CH₂)_(q)CH(CH₃)₂  <4> Provided that in the generalformula <4>, p is an integer selected from a range of 14 or more and 22or less, and q is an integer selected from a range of 1 or more and 3 orless.
 13. The magnetic recording medium according to claim 1, whereinthe base layer has a large number of holes, and the recesses of themagnetic layer are connected to the holes of the base layer.
 14. Themagnetic recording medium of claim 12, wherein in general formula <2>, asum of n and m is an integer selected from a range of 14 or more and 18or less.
 15. The magnetic recording medium of claim 12, wherein ingeneral formula <3>, p is an integer selected from a range of 14 or moreand 18 or less, and q is an integer selected from a range of 2 or moreand 4 or less.
 16. A magnetic recording/reproducing device comprising: afeeding unit that can sequentially feed out a tape-shaped magneticrecording medium; a winding unit that can wind up the magnetic recordingmedium fed out from the feeding unit; and a magnetic head that can writeinformation on the magnetic recording medium and can read outinformation from the magnetic recording medium while being in contactwith the magnetic recording medium traveling from the feeding unittoward the winding unit, wherein the magnetic recording medium includes:a substrate; a base layer disposed on the substrate; a magnetic layerdisposed on the base layer; and a back layer disposed on a side of thesubstrate opposite to the base layer, the substrate includes a polyesteras a main component, a surface of the back layer opposite to thesubstrate has a kurtosis of 2.0 or more, on a surface of the magneticlayer, recesses having a depth of 20% or more of the average thicknessof the magnetic layer are formed at a ratio of 10 or more and 200 orless per 1600 μm², a surface of the magnetic layer has arithmeticaverage roughness Ra of 2.5 nm or less, the base layer and the magneticlayer each include a lubricant, the entire magnetic recording medium hasa BET specific surface area of 3.5 m²/g or more in a state where thelubricant has been removed from the magnetic recording medium and themagnetic recording medium has been dried, a squareness ratio in aperpendicular direction is 65% or more, the magnetic layer has anaverage thickness of 90 nm or less, and the magnetic recording mediumhas an average thickness of 5.6 μm or less.
 17. The magneticrecording/reproducing device according to claim 16, wherein a tensionapplied to the magnetic recording medium in a longitudinal directionthereof can be adjusted.
 18. A magnetic recording medium cartridgecomprising: a tape-shaped magnetic recording medium; and a casing thathouses the magnetic recording medium, wherein the magnetic recordingmedium includes: a substrate; a base layer disposed on the substrate; amagnetic layer disposed on the base layer; and a back layer disposed ona side of the substrate opposite to the base layer, the substrateincludes a polyester as a main component, a surface of the back layeropposite to the substrate has a kurtosis of 2.0 or more, on a surface ofthe magnetic layer, recesses having a depth of 20% or more of theaverage thickness of the magnetic layer are formed at a ratio of 10 ormore and 200 or less per 1600 μm², a surface of the magnetic layer hasarithmetic average roughness Ra of 2.5 nm or less, the base layer andthe magnetic layer each include a lubricant, the entire magneticrecording medium has a BET specific surface area of 3.5 m²/g or more ina state where the lubricant has been removed from the magnetic recordingmedium and the magnetic recording medium has been dried, a squarenessratio in a perpendicular direction is 65% or more, the magnetic layerhas an average thickness of 90 nm or less, and the magnetic recordingmedium has an average thickness of 5.6 μm or less.